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.

Add custom templates for error messages in forms generated by django-material

See merge request AlekSIS/AlekSIS!147

Make primary group optional

See merge request AlekSIS/AlekSIS!152

Support global permission flags by LDAP group

Closes #178

See merge request AlekSIS/AlekSIS!149

Synchronise name and email to User object when saving a Person

See merge request AlekSIS/AlekSIS!151

Remove explicit naming of contributing organisations

See merge request AlekSIS/AlekSIS!150

It belongs on the project website.

Migrations

See merge request AlekSIS/AlekSIS!148

Use git version of django-material for this

Containers cannot update shared volumes from new image versions

This is to force update of static files if mounted from a host volume.