models: Add is_private flag to UserMessage and add index for it.

The is_private flag is intended to be set if recipient type is
'private'(1) or 'huddle'(3), otherwise i.e if it is 'stream'(2), it
should be unset.

This commit adds a database index for the is_private flag (which we'll
need to use it). That index is used to reset the flag if it was
already set. The already set flags were due to a previous removal of
is_me_message flag for which the values were not cleared out.

For now, the is_private flag is always 0 since the really hard part of
this migration is clearing the unspecified previous state; future
commits will fully implement it actually doing something.

History: Migration rewritten significantly by tabbott to ensure it
runs in only 3 minutes on chat.zulip.org.  A key detail in making that
work was to ensure that we use the new index for the queries to find
rows to update (which currently requires the `order_by` and `limit`
clauses).

scripts/lib/upgrade-zulip-stage-2

@@ -98,6 +98,7 @@ usermessage_index_migrations = [
     "[ ] 0095_index_unread_user_messages",
     "[ ] 0098_index_has_alert_word_user_messages",
     "[ ] 0099_index_wildcard_mentioned_user_messages",
+    "[ ] 0177_user_message_add_and_index_is_private_flag",
 ]
 # Our next optimization is to check whether any migrations are needed
 # before we start the critical section of the restart.  This saves