Fixing display error for separate 'log-in/sign-up'
Merge pull request #643 from andrewhayward/tweaks

Fixing display error for separate 'log-in/sign-up'
several small fixes
Merge pull request #646 from andrewhayward/auth

Using `FieldError` for unavailable username
Overhauling user sign-up flow

Allowing users to restart sign-up, and go back in the process should they need to.
Fixes second item in #621
Adding 'go back/restart' buttons to sign-up forms