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.
We need a migration to clear the tutorial_status for existing users,
so that we don't show hotspots to anyone who signed up for Zulip in
the month or so since we deleted the old tutorial.