allauth: Remove now-unused EMAIL_SEND_HTML setting

The setting was used in our own fork of django-registration exclusively.
allauth supports html emails either way, so we don't need this anymore
allauth: Use allauth setting for email confirmation expiry
allauth: Drop now-unused password reset urls
allauth: Move pootle_profile urls to /profiles/
allauth: Use accounts/ as the base url for allauth
allauth: Drop registration app
allauth: Update requirements