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:

- [x] Fix tests

- [x] 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

- [x] Open kuma-lib pull request: https://github.com/mozilla/kuma-lib/pull/47
Not at all, although we we need to use `django.core.urlresolvers.reverse_lazy` here since the settings module is evaluated at the start of the Django process which makes using reverse set the URL in stone, instead of allowing the URL resolve make it flexible (e.g. for the locale prepending).