Closes#38872
Labels in the label selection dropdown (issue/PR sidebar, new issue
form) were always listed alphabetically, so a scoped set like the
default Priority labels showed up as Critical, High, Low, Medium even
though each label carries an exclusive order.
This adds `CompareLabelForDisplay`/`SortLabelsForDisplay` in
`models/issues`: labels are grouped by their exclusive scope and sorted
by exclusive order within a scope (unordered ones last), falling back to
name order. The sorting is applied to the issue page sidebar data and
the shared label filter data, so the filter dropdown on the issue list
gets the same ordering.
Unscoped labels are unaffected and still sort by name. Includes a unit
test covering the default Priority label set.
