Allow autocompleting labels using colons and spaces

What does this MR do?

Fixes #56510 (closed) and #63343 (closed)

This MR allows auto-completing scoped and multi-word labels using the colon key. Previously, typing : or <space> exited the autocomplete dialog.

Caveats

• When auto-completing a label containing a colon or space, it is necessary to <enter> or <tab> to accept the autocomplete. <space> will not accept the autocomplete, and the label will not be valid. For single-word labels, this is not necessary, because the auto-completion isn't technically applied but the Markdown parser picks up on single-word labels anyway.
  • Basically, ~oneword is recognized by the Markdown parser as a label, but ~one word is not. The autocomplete transforms it to ~"one word".
  • I think this is actually a good thing, because if pressing the spacebar accepted autocompletes, you'd end up with ambiguous situations if you have labels like Next and Next release.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers