Allow filtering users on backend in RTE

What does this MR do and why?

Allow filtering users on backend in RTE

Changelog: changed

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The search param is passed to the backend if filterOnBackend is true. Note that the search results still include all people who have commented on the issue, which is further filtered on the frontend. This is important for working with quick actions like /assign and /unassign.

How to set up and validate locally

1. In rich text editor in issues, type @ to trigger user autocomplete.
2. In devtools notice that the call to autocomplete has a search param included.

Related to #439095 (closed)