user_message: Rename unused flags and create an index.

This commit renames the two unused and historical bits of the
'fields' bitfield of the 'UserMessage' and 'ArchivedUserMessage'
tables.

* 'summarize_in_home' to 'topic_wildcard_mentioned'
* 'summarize_in_stream' to 'group_mentioned'

The 'group_mentioned' flag doesn't affect the feature,
but completing the work here helps to save future migration
and indexing efforts on the UserMessage table, as we plan to
use this flag in the future for group mentions.

The unused bits may have old data; we'll clear that in
a separate commit.

It creates the 'zerver_usermessage_any_mentioned_message_id'
index concurrently.

zerver/lib/test_classes.py

@@ -2277,7 +2277,7 @@ one or more new messages.
         return body
 
 
-class MigrationsTestCase(ZulipTestCase):  # nocoverage
+class MigrationsTestCase(ZulipTransactionTestCase):  # nocoverage
     """
     Test class for database migrations inspired by this blog post:
        https://www.caktusgroup.com/blog/2016/02/02/writing-unit-tests-django-migrations/
@@ -2295,6 +2295,7 @@ class MigrationsTestCase(ZulipTestCase):  # nocoverage
 
     @override
     def setUp(self) -> None:
+        super().setUp()
         assert (
             self.migrate_from and self.migrate_to
         ), f"TestCase '{type(self).__name__}' must define migrate_from and migrate_to properties"