migrate djangocms-bootstrap5 to djangocms-frontend

as a pre-requisite to !175 (merged), this MR migrates from djangocms-bootstrap5 to djangocms-frontend as the former was never really maintained. The migration can be done with python manage.py frontend migrate but the following setting must be included in settings.py:

DJANGOCMS_FRONTEND_ADDITIONAL_MIGRATIONS = [
    "academic_community.management.bootstrap5_migration"
]