Added audit user for frontend groundwork that mimics an external user

What does this MR do?

Adds the frontend involved in audit users.

This involves a lot of backend to get it working, so the audit users currently mimic external users regarding privilege.

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

I've left TODO comments all over this for backend to check as areas that I think need to be updated. A full check should be done anyway, there should also be some better specs added once it all works as expected.

Why was this MR needed?

Screenshots (if relevant)

Show user

Audit index filter

User form

Application setting

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?

Part of #19737 (closed)