Always allow standalone login

[anandthakker]

Enable plain Django account logins via /accounts/standalone-login/, even when an allauth provider (e.g. Google) is enabled.

Create Django users with:

user@host> manage.py shell
>>> from django.contrib.auth.models import User
>>> user=User.objects.create_user('user_name', password='password')
>>> user.email = 'email@somewhere.com'
>>> user.save()