change_subdomain: Create a deactivated realm on updating subdomain.

When changing the subdomain of a realm, create a deactivated realm with
the old subdomain of the realm, and set its deactivated_redirect to the
new subdomain.
Doing this will help us to do the following:
- When a user visits the old subdomain of a realm, we can tell the user
that the realm has been moved.
- During the registration process, we can assure that the old subdomain
of the realm is not used to create a new realm.

If the subdomain is changed multiple times, the deactivated_redirect
fields of all the deactivated realms are updated to point to the new
uri.

analytics/tests/test_views.py

@@ -745,7 +745,8 @@ class TestSupportEndpoint(ZulipTestCase):
         realm_id = lear_realm.id
         lear_realm = get_realm('new-name')
         self.assertEqual(lear_realm.id, realm_id)
-        self.assertFalse(Realm.objects.filter(string_id='lear').exists())
+        self.assertTrue(Realm.objects.filter(string_id='lear').exists())
+        self.assertTrue(Realm.objects.filter(string_id='lear')[0].deactivated)
 
         result = self.client_post("/activity/support", {"realm_id": f"{lear_realm.id}", "new_subdomain": "new-name"})
         self.assert_in_success_response(["Subdomain unavailable. Please choose a different one."], result)
@@ -753,6 +754,9 @@ class TestSupportEndpoint(ZulipTestCase):
         result = self.client_post("/activity/support", {"realm_id": f"{lear_realm.id}", "new_subdomain": "zulip"})
         self.assert_in_success_response(["Subdomain unavailable. Please choose a different one."], result)
 
+        result = self.client_post("/activity/support", {"realm_id": f"{lear_realm.id}", "new_subdomain": "lear"})
+        self.assert_in_success_response(["Subdomain unavailable. Please choose a different one."], result)
+
     def test_downgrade_realm(self) -> None:
         cordelia = self.example_user('cordelia')
         self.login_user(cordelia)