Uses "sticky" positioning to make sure the x-axis is always visible even
when the SVG overflows the container. The main SVG's position has been
changed to "relative" to shift up the bottom based on the x-axis height.

The measurements panel now expects each collection to have an array of
threshold values.

This change is backwards compatible because single value thresholds are
converted to an array during the loading of the measurements JSON.
Note `collection.thresholds` takes precedence over the deprecated
`collection.threshold`, so the single value will be ignored if
`collection.thresholds` exists.

All threshold values will be displayed as solid grey lines across the
subplots. The toggle for showing thresholds applies to all threshold
values. Future improvements include allowing customizations of
solid vs dashed lines and individual toggles for each threshold.

For each subplot, jitter the y-values of the raw measurements within
each color-by group. The y-value range for each color-by group is
determined by the proportion of measurements within each group, i.e.
color-by groups with more measurements get a larger portion of the
subplot height.

Removes the jitter value added during loading of the measurements JSON
and the `measurementJitterSymbol` since they are no longer needed.

Testing to see if lowering the opacity of the raw measurement circles
can help with visualizations of denser areas of the plots.

Restores the behavior prior to adding the sticky x-axis where the
container resets to the top when the SVG is redrawn.

This explicit reset fixes the issue where the sticky x-axis can
potentially leave the panel in a scrolled state that displays a
huge white space.¹

¹ #1631 (comment)