add_forge_now: Rename django app and add missing app_label

This fixes errors when using django 3.2 and thus the packaging on debian unstable

See https://jenkins.softwareheritage.org/job/debian/job/packages/job/DWAPPS/job/gbp-buildpackage/405/console

Related to T4061

---

Migrated from D7395 (view on Phabricator)