Fix assignees search to show only project members

What does this MR do?

Currently, when searching on issue assignees, results show all the GitLab users. This MR limit rendered search results to project members that match the search term and issue participants that match the search term

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability 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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Related to #322960 (closed)