https://github.com/jazzband/django-constance/pull/359

Replace django-dbsettings with django-constance

See merge request AlekSIS/AlekSIS!96

Rationale: django-constance is a bit better maintained and has some
features we need, e.g. injecting config into templates through a context
processor. Also, decoupling the settings definitions from import time
allows us to implement lazy access to the config, so we can later access
them in other variables insettings.py directly.

Resolve "Allow defining imprint and privacy policy URLs as dbsetting"

Closes #128

See merge request AlekSIS/AlekSIS!93

Copy messages from SchoolApps' header template to AlekSIS base template

Closes #127

See merge request AlekSIS/AlekSIS!95

Close #127

- 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

WTF poetry?!

Merge models of SchoolApps and BiscuIT

See merge request AlekSIS/AlekSIS!92

BiscuIT → AlekSIS; mention build-essential as necessary

See merge request AlekSIS/AlekSIS!91

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

Close "Recreate migrations" from #134

Rename BiscuIT-ng to AlekSIS

See merge request AlekSIS/AlekSIS!90