Fix group and project search for anonymous users

What does this MR do?

Enables the filter dropdowns on search for anonymous users.

This allows users to search on the page like a logged-in user would, and it also allows the group_id and project_id to be preserved between searches by anonymous users, so making the search experience vastly more usable.

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

We make GET /api/v4/groups and GET /api/v4/groups/:id available to unauthenticated users. The equivalent project endpoints are already available to unauthenticated users, so I think this is fine.

Group search undergoes a bit of refactoring. I've checked the places where GroupsFinder is used and it all seems fine to me, but it could do with another once-over to be sure.

Why was this MR needed?

Without this, searching in a project takes you to a page that will do a global search if you try to refine your search at all, which is extremely jarring.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • [ ] Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #31409 (closed)