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).

zerver/management/commands/create_large_indexes.py

@@ -88,6 +88,14 @@ def create_indexes() -> None:
         where_clause='WHERE (flags & 8) != 0 OR (flags & 16) != 0',
     )
 
+    # copied from 0177
+    create_index_if_not_exist(
+        index_name='zerver_usermessage_is_private_message_id',
+        table_name='zerver_usermessage',
+        column_string='user_profile_id, message_id',
+        where_clause='WHERE (flags & 2048) != 0',
+    )
+
 class Command(ZulipBaseCommand):
     help = """Create concurrent indexes for large tables."""