Update dependency Pillow to v8

See merge request !393

Update dependency Celery to v5

See merge request !392

Update dependency django-celery-results to v2

See merge request !401

Update dependency importlib-metadata to v3

See merge request !402

Resolve "Replace django-easy-audit"

Closes #319

See merge request !385

# Conflicts:
#   poetry.lock
#   pyproject.toml

Resolve "Fix connector mapping for dbbackup after prometheus installation"

Closes #321

See merge request !399

Resolve "Correct Mixins"

Closes #309

See merge request !397

As the image cropping field will later be removed, it is ok to create
a dummy in the initial migration.

Installations that already migrated with the original image cropping
field are untouched because they already have the migration applied;
new installations migrating from scratch will get the dummy and later
replace it.