Add better error reporting to users for password login / signup errors

Closes #26.
Fix HTML-formatted email messages
Prevent non-password users from logging in using the password mechanism
Merge branch 'password_form_errors'

* password_form_errors:
  Prevent non-password users from logging in using the password mechanism
  Fix HTML-formatted email messages
  Add better error reporting to users for password login / signup errors
Merge branch 'master' of github.com:SlideAtlas/SlideAtlas-Server into dev/dj-websocket-performance