Add filtering to `/autocomplete_sources/members`

In https://gitlab.com/gitlab-org/gitlab-ce/issues/37272 we want to use the /autocomplete_sources/members endpoint (this includes the current noteable's author and participants) to load the users for the assignee dropdown so that it is consistent with the autocomplete in the description / comments.

We would need to add support for filtering by name / username because the assignee dropdown supports lazy-loading based on input. This would also be useful when we implement https://gitlab.com/gitlab-org/gitlab-ce/issues/46423

We might also want to implement a param to limit the results. The current endpoint used by the dropdown is hardcoded to return 20 results only.

As noted in https://gitlab.com/gitlab-org/gitlab-ce/issues/37272#note_206990374, we might also need to add some fields in the returned JSON.