upgrade to django5 and django-bootstrap5

@renovate-bot correctly identified that we run an old version of django (4.2) which is still supported, but will be EOL in 2026 (april 30th). Django 5.2 is the new LTS, supported until 2028, so we need to upgrade django some time in early 2026, ideally before the spring campaign.

@renovate-bot also identified django-bootstrap-v5 as a problematic dependency, because it's incompatible with django 5 (and, anyways, dead upstream), so a lot of the work here will be to port to that new plugin.