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 (ongoing..)

- [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
I feel like this shouldn't just be dropped as it shows an actual problem of our i18n system. I'll mark it with a `SkipTest` exception so we can look at it when we purge sumo from the code.