Scoped label improvements

[brechtvl]

Feature Description

Now that there is an initial implementation of scoped labels (#22585), there are some things to improve. Aimed at v1.19:

• Fix problem where long label name causes label to break across two lines (Scoped label display and documentation tweaks #23430)
• Don't rely on alt key for removing scoped labels from the label and filter assignment menus, as this can conflict with operating system or browser bindings. (Scoped labels: don't require holding alt key to remove #23303)
• Documentation for this feature. (Add basic documentation for labels, including scoped labels #23304)
• Find on a11y solution for graying out Exclusive checkbox when no valid name is set. It is convenient to be able to still do it when / was removed from the label name, however there is no communication for blind users that the exclusive checkbox is grayed out. (Scoped labels: set aria-disabled on muted Exclusive option for a11y #23306)

Important (for v1.20?):

• Render non-exclusive scoped labels in the same style (Scoped label display improvements #23164)
• Automatically remove conflicting scoped labels on all existing issues, when marking labels as exclusive.
  • Ideally it would display the number of labels that will be removed beforehand. This might be fairly expensive to compute for all labels when opening the labels page, so might need an AJAX request. Perhaps not critical for an initial solution.

Nice to haves:

• Convenient scoped label switching by clicking on the label itself, and showing a menu with other labels in the same scope.
• Nested scoped labels could have some custom rendering, currently they still show the / in the text.
• Avoid overflow when label contains long words (see Scoped label display improvements #23164 for details)

[lafriks]

Created PR #22976 to allow creating labels from yaml template files that allows exclusive scope option set

[lafriks]

One thing I noticed (could be bug?) - non-exclusive scoped labels should also be rendered same as exclusive

[brechtvl]

Communicating which labels are exclusive seems important. So even if we render non-exclusive ones in a similar style, there should probably be some visual difference still? I don't immediately have a good idea for what that would look like.

[lafriks]

I don't think it's important to see it's exclusive or not when displaying issue or in issue list. It's only important when displaying label selection menu where it's already understandable which are exclusive and which are not