Advances #165.

ExtensibleModel is now an abstract base model.

- mail out name and address, impress and privacy policy URL
- Move dbsettings to own file to prevent importing in circle
- Add context processor to provide dbsettings in templates
- Add "Katharineum zu Lübeck" in footer menu

Close #128

Merge models, remove old files, register Activity and Notification in admin interface and recreate migrations

Close "Recreate migrations" from #134

Additionally change icon of admin menu entry

Additionally move all CSS to style.scss and remove unnecessary db settings

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

TIL: Settings instance always needs a verbose_name

Advances #115.

Instead, CRUDMixin is now available to replace the crud_events relation
that was previously provided by SchoolRelated.

Advances #69.