User creation form needs feedback, validation & error messages

[nsreed]

• Validation
  • User exists
  • Passwords don't match
• Disable submit button when form submitted
• Progress spinner
• Connection/GUN error

[ultimape]

Does not do the right thing when offline, so the "Connection/GUN error" state does not seem to trigger. It just hangs.

[nsreed]

This seems to be caused by a ChunkLoadError. Which makes sense, since the app can't load a chunk when the server's offline. Yet another reason we need to hurry up and implement the service worker.