Switch to allauth for Persona, prepare Github integration

This is the next step to enabling Github login. It throws away all of
the django-browserid and custom code and replaces it with allauth's support
for Persona.

Here are the current TODOs for this:

[ ] Fix tests

[ ] Double and triple check migrations for email adresses and social accounts

[ ] Extend documentation

[ ] Update more allauth templates to use our base template blocks and variables

[ ] Open kuma-lib pull request