* Labels now consistently have the same shape, emojis and tooltips everywhere.
  This includes the label list and label assignment menus.
* In label list, show description below label same as label menus.
* Don't use exactly black/white text colors to look a bit nicer.
* Simplify text color computation. There is no point computing luminance in
  linear color space, as this is a perceptual problem and sRGB is closer to
  perceptually linear.
* Increase height of label assignment menus to show more labels. Showing only
  3-4 labels at a time leads to a lot of scrolling.
* Render all labels with a new RenderLabel template helper function.

* Change label creation to open a modal menu like label editing.
* Change menu layout to place name, description and colors on separate lines.
* Don't color cancel button red in label editing modal menu.
* Align text to the left in model menu for better readability and consistent
  with settings layout elsewhere.

Add a new "exclusive" option per label. This makes it so that when the label is
named `scope/name`, no other label with the same `scope/` prefix can be set on
an issue.

The scope is determined by the last occurence of `/`, so for example
`scope/alpha/name` and `scope/beta/name` are considered to be in different
scopes and can coexist.

Exclusive scopes are not enforced by any database rules, however they are
enforced when editing labels at the models level, automatically removing any
existing labels in the same scope when either attaching a new label or
replacing all labels.

In menus use a circle instead of checkbox to indicate they function as radio
buttons per scope. Issue filtering by label ensures that only a single scoped
label is selected at a time. Clicking with alt key can be used to remove a
scoped label, both when editing individual issues and batch editing.

* Display scoped label prefix and suffix with slightly darker and lighter
  background color respectively, and a slanted edge between them similar to
  the `/` symbol.
* In menus exclusive labels are grouped with a divider line.

…migrations

Add exclusive label migration

…warning

about what will happen with conflicting scoped labels.