This is required because our migration is going to go in two phases.
When we do the database migration (on pushing to master), we update
all messages at that point. But prod doesn't know about the new
flags field, so any new messages sent on prod will not have the
read bit set.
When we push to prod, we want to re-run the bit of the migration script
that automatically sets read flags on messages older than the users's
pointer.
(imported from commit 961d33e972eac9ada80089bf1b1269c7fb42d56b)
Leo Franchi