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).