auth: Rewrite data model for tracking enabled auth backends.

So far, we've used the BitField .authentication_methods on Realm
for tracking which backends are enabled for an organization. This
however made it a pain to add new backends (requiring altering the
column and a migration - particularly troublesome if someone wanted to
create their own custom auth backend for their server).

Instead this will be tracked through the existence of the appropriate
rows in the RealmAuthenticationMethods table.

zerver/tests/test_slack_importer.py

@@ -734,7 +734,7 @@ class SlackImporter(ZulipTestCase):
             passed_realm["zerver_realm"][0]["description"], "Organization imported from Slack!"
         )
         self.assertEqual(passed_realm["zerver_userpresence"], [])
-        self.assert_length(passed_realm.keys(), 15)
+        self.assert_length(passed_realm.keys(), 16)
 
         self.assertEqual(realm["zerver_stream"], [])
         self.assertEqual(realm["zerver_userprofile"], [])
@@ -1148,6 +1148,10 @@ class SlackImporter(ZulipTestCase):
 
         self.assertEqual(Message.objects.filter(realm=realm).count(), 82)
 
+        # All auth backends are enabled initially.
+        for name, enabled in realm.authentication_methods_dict().items():
+            self.assertTrue(enabled)
+
         Realm.objects.filter(name=test_realm_subdomain).delete()
 
         remove_folder(output_dir)