Place the all mention autocompletion behind FF

What does this MR do and why?

For task #415523 (closed) and issue #18442 (closed).

When disable_all_mention is enabled @all should not appear in the autocompletion lists.

How to set up and validate locally

Visit the autocompletion route with/without the feature flag disable_all_mention enabled:

For project autocompletion:

http://<your gdk addr>/gitlab-org/gitlab-test/-/autocomplete_sources/members?type=Issue&type_id=3

For group autocompletion:

http://<your gdk addr>/groups/gitlab-org/-/autocomplete_sources/members?type=Epic&type_id=1

When the FF is enabled, expect to not see the @All item. The sorting of the autocompleted items happen on the frontend so you might have to search for a second or two. For the project autocompletion route, name should be All Project and Group Members.

- {
- "username": "all",
- "name": "All Group Members",
- "count": 11
- },

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.