allauth: Use allauth for registration and login page

NOTE: The new functionality does not implement legal pages on
registration, however it did not work properly before either...

NOTE 2: Interface Language is dropped. See bug #3179
allauth: Add LOGIN_REDIRECT_URL to root by default
allauth: Use allauth for password change form
allauth: Use allauth for password reset
allauth: Remove duplicate error from login page
allauth: Use allauth for email confirmation
allauth: Use allauth for closed signups
allauth: Drop an unused template
allauth: Move the last template to account/
allauth: Remove now-unused EMAIL_SEND_HTML setting
allauth: Use allauth setting for email confirmation expiry
allauth: Drop now-unused password reset urls
allauth: Move pootle_profile urls to /profiles/
allauth: Fix a leftover hardcoded url
allauth: Use accounts/ as the base url for allauth
allauth: Drop registration app
allauth: Update requirements