WIP: Check username in use during registration before submit

What does this MR do?

Adds the /u/:username/exists endpoint and adds a debounced request on new user username input fields to request this endpoint to see if a username is in use.

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

Happy with error message?

Why was this MR needed?

Pre-membership UX. 😃

What are the relevant issue numbers?

Closes #5991 (closed).

Screenshots (if relevant)