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.

We must ensure we only use supported backends, but as this contains
PostgreSQL, MySQL and SQLite, I see no blocker there.

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)