user_groups: Move constants for system group names to a new class.

This commit moves constants for system group names to a new
"SystemGroups" class so that we can use these group names
in multiple classes in models.py without worrying about the
order of defining them.

zerver/lib/test_classes.py

@@ -95,6 +95,7 @@ from zerver.models import (
     Recipient,
     Stream,
     Subscription,
+    SystemGroups,
     UserGroup,
     UserGroupMembership,
     UserMessage,
@@ -1350,7 +1351,7 @@ Output:
             realm, invite_only, history_public_to_subscribers
         )
         administrators_user_group = UserGroup.objects.get(
-            name=UserGroup.ADMINISTRATORS_GROUP_NAME, realm=realm, is_system_group=True
+            name=SystemGroups.ADMINISTRATORS, realm=realm, is_system_group=True
         )
 
         try: