cicd: PR checks should look to see if the PR changes models without creating corresponding migrations

[rjsparks]

The old mkrelease script had this check:

note "Checking that all model changes have been captured in migrations ..."
$do ietf/manage.py makemigrations | tee /dev/stderr | $do grep -q "^No changes detected$" || die "Model changes without migrations found."

We should have something similar in the tests that run over PRs, and again when cutting releases.