Make frequencies tend to 0 in absence of data

The schema currently allows datasets to provide a scale for non-
continuous scales where specific trait values are given colour hexes
(missing values are given greys by auspice).

Here we extend this to continuous scales by interpreting the same
data structure as anchor points which we interpolate between using
the same method as we currently use for generating default continuous
color scales (d3's `interpolateRgb`)

This allows continuous colour scales to define custom legend
entries, via a `legend` key in the JSON. This allows control
over the values in the scale which we use as legend elements,
the displayed text, and the range of values which each entry covers.

Bounds are enforced to be non-overlapping. If overlapping bounds
are detected, we revert to Auspice dynamically generating these.
(This is a requirement for future work which will map continuous
tip values to a legend entry, which will allow pie-chart display
using the legend swatches.)

This restores the algorithm used to associate a hovered legend
item to tips for continuous variables. Commit
0f37b1a (Mar 2018) incorrectly changed
this to `tip \in [a, b]` rather than the intended (and documented)
`tip \in (a, b]`.

This takes on more importance given that the previous commit allows
user-defined bounds.

Note that the frequencies panel already used `(a, b]` matching, so now
the legend matching mirrors this.

…ements

Continous color scale improvements

This action will run on each auspice PR and create a corresponding
PR on nextstrain.org which includes a commit using the version
of auspice from this (auspice) PR. This functionality is extremely
useful for auspice development as it will allow us to use a Heroku
review app to test auspice in the context of nextstrain.org

There are a number of future improvements to implement:

* New auspice releases (tagged commits on `release` branch)
would ideally create a PR on nextstrain.org which could be merged
to update the version of auspice there.

* Other consumers of auspice (e.g. auspice.us) could be added to this
GitHub Action.

GitHub Action to create nextstrain.org PR

This implements a requested improvement to the original
scatterplot implementation. The implementation hinges on two changes:
(1) The collection of values for a given variable (e.g. x-var) need
to be computed and passed to PhyloTree to act as the scale's domain.
We reuse the colorScale machinery here, which could be optimised
(see todo messages in code), but this has the advantage that the
domain ordering matches the legend (unless user supplied).
(2) PhyloTree needed to be modified to use non-linear scales, in this
case `pointScale`.

This commit should be fully functional, however there are some
future improvements to be made:

(i) Grid text is obscured and unreadable when there are many entries
in the domain.
(ii) Genotypes and Boolean scales are not yet available.
(iii) Jitter should be added to nodes to avoid obfuscation.

This commit is in preparation for allowing genotype to be a scatterplot
variable. This will complicate the allowable scatterplot variables
and force these to update upon colorBy changes. This is much cleaner
if layout is changed in a thunk.

Genotype is treated differently to other colorings in two important
ways: (1) it can change value, for instance when changing the
colorby to another genotype position and (2) it is stored in a
different place to other colorings. These require scatterplot logic
to be more complex as actions are no longer separate - we now require
a NEW_COLOURS action to potentially update the layout which was
formerly within the remit of the CHANGE_LAYOUT actions. This is
achieved through a middleware layer.

This implementation makes it clear that jitter and better domain
spacing are crucial for scatterplots.

This prevents nodes falling on the axis itself or at the very end of
the grid, which was especially noticeable for traits with small domains.

Allow non-continuous scatterplot variables

We have had issues in the past with legend values from column 1 overflowing into column 2. For instance, issue #899 was fixed by PR #914 which implemented a maximum character limit for legend names. This solution can produce misleading views, such as those described in #1306.

This solution implements a clipping mask for column 1, avoiding the complication of limiting the string size. Column 2 already has similar behaviour because the SVG element of the legend itself performs the clipping.

Apply clipping to first column of legend

Use filterOptions to modify filter search algorithm

Fixes a bug where the ability to toggle regression lines was hidden for clock views. (The ability to hide this toggle is only intended for scatter layouts, where we should not expose the toggle unless both axes are showing continuous variables.)

Always show regression toggle for clock layout

The existing grayscale color ramp (used for values absent in an explicitly specified color scale) had values that were too dark and threw off the overall color balance. This commit narrows the grayscale color ramp to be more in line with pastel color ramp.

This adds a bit of blue into the grayscale color ramp. Still reads as mostly gray, but no colors seem to exist more in the same universe as canonical auspice color ramp.

Adjust grayscale color ramp

Styling adjustments to footer text

This commit uses dataProvenance in metadata to identify datasets using "GISAID" data. For these datasets, the full metadata download is swapped to an "acknowledgments" download that only includes the following fields:
 - strain
 - gisaid_epi_isl
 - genbank_accession
 - originating_lab
 - submitting_lab
 - author

This commit cleans up naming of metadata headers in downloaded metadata TSV. It does the following:
1. Keeps headers as input into "augur export" rather than renaming by title. Thus it has "originating_lab" rather than "Originating lab", "pango_lineage" rather than "PANGO lineage", etc... This should make it easier for people to process downloaded metadata from Auspice alongside metadata provisioned by Nextstrain (via GISAID or via S3).
2. Makes "date" the second column as this is often what's most important. I couldn't figure out a way to intelligently order remaining fields. My first thought was to use metadata.colorings, but this isn't sorted.
3. Fixes "accession". It had been exporting as "[object Object]".

Remove metadata download from GISAID datasets

Cleanup metadata headers