This enables admins to customize their landing / home page via the Admin
dashboard -- including the text at the top of the page and the section
below it. It keeps the current default text, falling back to it if the
user hasn't overwritten it.
Ref T565
- Add server-side validation
- Normalize usernames
- Check username existence when signing up
- Tailor UI to whether federation is enabled or not
- Return user to form and show errors on sign up
