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

mayan/apps/user_management/admin.py

@@ -0,0 +1,17 @@
+from django import forms
+from django.contrib import admin
+from django.contrib.auth.models import Group
+from django.contrib.auth.admin import UserAdmin
+from django.contrib.auth.forms import ReadOnlyPasswordHashField
+
+from user_management.models import MayanUser
+
+
+@admin.register(MayanUser)
+class MayanUserAdmin(UserAdmin):
+    list_display = ('organization',) + UserAdmin.list_display
+    list_display_links = ('username',)
+    list_filter = UserAdmin.list_filter + ('organization',)
+    ordering = ('organization',) + UserAdmin.ordering
+    fieldsets = UserAdmin.fieldsets
+    fieldsets[1][1]['fields'] = ('organization',) + fieldsets[1][1]['fields']