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
To be honest, I don't know why we have that pattern at all, we don't use it anywhere and Google Analytics doesn't have any data about it (I guess because it's a redirect?).