- Add django-pwa to pyproject.toml
- Add configuration for django-pwa
- Include PWA configuration and favicon support in base.html
- Include serviceworker.js and adapt it for AlekSIS
- Remove old files

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.

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

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

Close "Recreate migrations" from #134