Currently translated at 14.6% (53 of 363 strings)

Translation: AlekSIS/AlekSIS-Core
Translate-URL: https://translate.edugit.org/projects/aleksis/aleksis/la/

Currently translated at 100.0% (363 of 363 strings)

Translation: AlekSIS/AlekSIS-Core
Translate-URL: https://translate.edugit.org/projects/aleksis/aleksis/de/

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.

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)

This reverts merge request !339

Also move initialization of date and time pickers to own functions