This avoids risk of OOM issues on servers with relatively limited RAM
and millions of messages of history; apparently, fetching all messages
ordered by ID could be quite memory-intensive even with an iterator
usage model.
Fortunately, we have other migrations that already follow this pattern
of iterating over messages, so it's easy to borrow existing code to
make this migration run reasonably.
This fixes a few minor issues with the migration:
* Skips messages with empty rendered_content, fixing an exception that
affected 4 messages on chat.zulip.org.
* Accesses messages in order.
* Provides some basic output on the progress made.
This should make life substantially better for any organizations that
run into trouble with this migration, either due to it taking a long
time to run or due to any new exceptions.
Tim Abbott