user_message: Use INSERT ... ON CONFLICT for historical UM creation.

Rather than use a bulk insert via Django, use the faster
`bulk_insert_all_ums` that we already have.  This also adds a `ON
CONFLICT` clause, to make the insert resilient to race conditions.

There are currently two callsites, with different desired `ON
CONFLICT` behaviours:
 - For `notify_reaction_update`, if the `UserMessage` had already been
   created, we would have done nothing to change it.
 - For `do_update_message_flags`, we would have ensured a specific bit
   was (un)set.

Extend `create_historical_user_messages` and `bulk_insert_all_ums` to
support `ON CONFLICT (...) UPDATE SET flags = ...`.

zerver/actions/message_flags.py

@@ -338,8 +338,9 @@ def do_update_message_flags(
 
             create_historical_user_messages(
                 user_id=user_profile.id,
-                message_ids=historical_message_ids,
-                flags=(DEFAULT_HISTORICAL_FLAGS & ~flagattr) | flag_target,
+                message_ids=list(historical_message_ids),
+                flagattr=flagattr,
+                flag_target=flag_target,
             )
 
         to_update = UserMessage.objects.filter(