Add better harness for updating `django-perf-rec` snapshots

[jmduke]

It's kind of annoying to deal with performance snapshot updates b/c django-perf-rec does not expose an "update snapshot" mechanism like pytest-snapshot does.

So let's do three things:

1. Mark every single test that uses django_perf_rec as @pytest.mark.db_snapshot. (We might be able to DRY this a bit by creating a fixture that applies the mark.
2. Augment app/update_snapshots to delete all .perf.yml files and then run just files tagged db_snapshot.

   ### Tasks

[catdevnull]

Mark every single test that uses django_perf_rec as @pytest.mark.db_snapshot. (We might be able to DRY this a bit by creating a fixture that applies the mark.

would this be by making a very simple perf_record or django_perf_rec fixture that just passes the record function?

[catdevnull]

FAILED emails/views/subscriber_facing/email--test.py::test_performance_with_a_handful_of_queries_without_billing - TypeError: run_performance_snapshot() missing 1 required positional argument: 'perf_record'
FAILED emails/views/subscriber_facing/subscribe--test.py::test_performance_with_a_handful_of_queries_without_billing - TypeError: run_performance_snapshot() missing 1 required positional argument: 'perf_record'
FAILED emails/views/subscriber_facing/email--test.py::test_performance_with_a_handful_of_queries_on_v2 - TypeError: run_performance_snapshot() missing 1 required positional argument: 'perf_record'

not sure what to do in these seemingly non-pytest tests