remove remainders of djangocms-bootstrap5

this is a follow-up on !176 (merged) and finishes the migration from djangocms-bootstrap5 to djangocms-frontend. Note that this is a breaking change! You first need to do the migration from !176 (merged) and the run implement the changes from this MR. Otherwise you may encounter data loss.

To run the migration,

1. install django-academic-community==0.0.1.dev0+282.g934a7def

2. add the following to your settings.py

   DJANGOCMS_FRONTEND_ADDITIONAL_MIGRATIONS = [
       "academic_community.management.bootstrap5_migration"
   ]

3. remove django.contrib.auth and django.contrib.admin from the INSTALLED_APPS in settings (they are now added via academic_community.INSTALLED_APPS)

4. run python manage.py migrate and python manage.py frontend migrate

afterwards you can continue with the changes introduced in this MR