What did I drink when I wrote the original code? The relative import
`from .. import __version__` in the utility code in core, of course,
always imported from `aleksis.core`, no matter the real `AppConfig`
that inherited the utility code. Thus, the versions always showed the
version of AlekSIS-Core everywhere (whose version information was
broken ever since we renamed AlekSIS to AlekSIS-Core).

This change decouples the `get_version` code from the package level
`__version__` information, which effectively becomes optional now.

Add method to inject permissions to ExtensibleModels dynamically

This change is primarily intended to allow models to declare their own
default data, without having to use data migrations. Data migrations
have the downside of being run only once, so e.g. `manage.py flush` will
delete default data wchich will never be restored. Also, I consider it
more natural todefine default data setup on model classes.

In order to achieve that, this adds a generic interface for apps to
register hooks called on several signals, by simply implementing methods
with the same name on their AppConfig instance.

This finalises the migration to the black code style by applying double-quotes.