Resolve "Filter issues input does not show assignee name when there is an assignee ID in the URL"

What does this MR do?

Resolves an issue in which using legacy assignee_id=123 or author_id=456 params instead of asignee_username=fred or author_id=tim would cause the issues to be filtered, but no asignee:@foo or author:@bar would show up in the filter search bar.

Are there points in the code the reviewer needs to double check?

I could have tackled this one of two ways:

1. Automatically redirect issues?assignee_id=123 to issues?assignee_username=fred in the Projects::IssuesController
2. Pass the id to username associations to the frontend via a data-attribute so the search bar can be generated correctly

I opted for the later. Let me know if this seems too hacky or if there is an alternative we should try.

Why was this MR needed?

see #26955 (closed)

Screenshots (if relevant)

before:

after:

Does this MR meet the acceptance criteria?

• [-] Changelog entry added
• [-] Documentation created/updated
• [-] API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #26955 (closed)