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 = ...`.
Anders Kaseorg
Alex Vandiver