Include leading http(s) on activation link.

Closes #961

allianceauth/authentication/templates/registration/activation_email.txt

@@ -2,7 +2,7 @@ You're receiving this email because someone has entered this email address while
 
 If this was you, please go to the following URL to confirm your email address:
 
-{{ url }}
+{{ scheme }}://{{ url }}
 
 This link will expire in {{ expiration_days }} day(s).
 

setup.py

@@ -17,7 +17,7 @@ install_requires = [
 
     'django>=1.11',
     'django-bootstrap-form',
-    'django-registration',
+    'django-registration>=2.4,<3.0',
     'django-sortedm2m',
     'django-redis-cache>=1.7.1',
     'django-celery-beat',