migration: Clear old data for unused usermessage flags.

In c37871ac3a, we renamed the
two unused and historical bits of the 'flags' bitfield of
the 'UserMessage' table:

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

This commit clears out the old data for those bits.

Additionally, we are clearing 'force_expand' and 'force_collapse'
unused flags to save future work.

zerver/models.py

@@ -3489,6 +3489,7 @@ class AbstractUserMessage(models.Model):
         "topic_wildcard_mentioned",
         "group_mentioned",
         # These next 2 flags are from features that have since been removed.
+        # We've cleared these 2 flags in migration 0486.
         "force_expand",
         "force_collapse",
         # Whether the message contains any of the user's alert words.