Pass search param in autocomplete_sources/members for rich text editor autocomplete suggestions

Description

#427437 (closed) solves the problem for interacting with users quickly in plain text editor. We should have this improvement in the rich text editor.

Proposal

Solve this problem for rich text editor in the same way under the same feature flag, for issues and MRs. MR for plain text suggestions: !139085 (merged)

Context from #427437 (closed):

Problem to solve

Currently, you need to wait a few seconds the first time you interact with the users dropdown (or quick actions).

Proposal

We can pass the search param to the backend autocomplete endpoint to only return results based on what user typed, with a limit of 20.

This change should be behind a feature flag gated by current_user.