Upgrade to django-allauth for authentication
Override login templates in core.
Update settings and title.
Remove registration templates.
Merge branch 'allauth' into merge-py3-django16

Conflicts:
	pip_requirements.txt
	readthedocs/templates/core/header.html
	readthedocs/templates/core/home-header.html
	readthedocs/templates/registration/activate.html
	readthedocs/templates/registration/activation_complete.html
	readthedocs/templates/registration/activation_email.txt
	readthedocs/templates/registration/logout.html
	readthedocs/templates/registration/password_reset_email.html
	readthedocs/templates/registration/registration_closed.html
	readthedocs/urls.py