feat(settings): add yaml-based settings loader

The new abstract configuration loader allows to configure a grouprise
site with fewer settings, since many related settings are deducted
automatically, e.g.:
* `domain` is sufficient for all mail addresses
* `transport_security` determines all SSL-related Django settings
* `template_directories` allows injection of local template directories
  (without cryptic manipulation of Django's `TEMPLATES[0]["DIRS"]`
3 jobs for feature/lars-yaml-settings in 4 minutes and 35 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #5863
make_deb_package

00:03:44

 
  Test
passed #5865
deb_package_test

00:00:50

passed #5864
make_test

00:01:22