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.

This reverts commit d21743fd.

Use setuptools entrypoints to find apps

See merge request !395

Update dependency celery-progress to ^0.0.14

See merge request !390

In contrast to scanning the `aleksis.apps` namespace, this has some
benefits:

 1. Does not need to import the module to consider it
    (resolve potential circular imports )
 2. Does not need to scan filesystems and wheels
 3. Apps can choose their own namespaces, register multiple apps/
    variants, and much more)

Use an installed django-admin script instead, e.g. `poetry run
django-adimn …`

Remove submodules and rename project

See merge request !386

Merge branch '317-separate-core-and-default-distribution' of edugit.org:AlekSIS/official/AlekSIS-Core into 317-separate-core-and-default-distribution

Use CI_COMMIT_TIMESTAMP instead of date %s.

See merge request AlekSIS/official/AlekSIS!388