Update authenthication code to support multitenants. Replace all remaining instances of hardcoded User model.

mayan/apps/authentication/auth/model_auth_backend.py

@@ -0,0 +1,20 @@
+from __future__ import unicode_literals
+
+from django.conf import settings
+from django.contrib.auth import get_user_model
+from django.contrib.auth.backends import ModelBackend
+
+
+class UsernameModelBackend(ModelBackend):
+    def authenticate(self, username=None, password=None, **kwargs):
+        UserModel = get_user_model()
+        if username is None:
+            username = kwargs.get(UserModel.USERNAME_FIELD)
+        try:
+            user = UserModel.on_organization.get(username=username)
+            if user.check_password(password):
+                return user
+        except UserModel.DoesNotExist:
+            # Run the default password hasher once to reduce the timing
+            # difference between an existing and a non-existing user (#20760).
+            UserModel().set_password(password)