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

mayan/apps/user_management/models.py

@@ -0,0 +1,20 @@
+from __future__ import unicode_literals
+
+from django.contrib.auth.models import AbstractUser, UserManager
+from django.core.urlresolvers import reverse
+from django.db import models
+from django.utils.encoding import python_2_unicode_compatible
+from django.utils.translation import ugettext_lazy as _
+
+from organizations.models import Organization
+from organizations.managers import CurrentOrganizationManager
+from organizations.shortcuts import get_current_organization
+
+
+class MayanUser(AbstractUser):
+    organization = models.ForeignKey(
+        Organization, default=get_current_organization
+    )
+
+    objects = UserManager()
+    on_organization = CurrentOrganizationManager()