This is a preparatory commit for using isort for sorting all of our
imports, merging changes to files where we can easily review the
changes as something we're happy with.
These are also files with relatively little active development, which
means we don't expect much merge conflict risk from these changes.
This fixes a performance issue that caused this migration to run for a
really long time.
It still takes about 1 minute to run with the 75K Subscription objects
we have on chat.zulip.org, but that's within the realm of acceptable.