Meta issue: change accounts stuff

[elnappo]

This includes

• #210 - Allow email login
• #47 - 2factor auth
• #76 - email address update needs roundtrip verification
• #120 - consider replacing django-registration
• #182 - add language to user profile, use it to translate email content

These problems could be fixed by changing some of the underlying login mechanismes. Were currently using django-registration-redux and python-social-auth. #210, #76, #120 could be fixed by switching to django-allauth. #47 with django-two-factor-auth.

One problem with django-allauth could be the migration of social-auth logins. Besides this with django-two-factor-auth it is not possible to use email logins.

I would suggest changing all these together.

[ThomasWaldmann]

Duplicate of #120?