This reverts merge request !261

Close #287

This reverts commit 53d82e48.

This reverts merge request !271

This MR includes:
- Create group types
- Edit group types
- Delete group types
- Connect group type to groups

This MR includes:
- List additional fields
- Create additional fields
- Edit additional fields
- Delete additional fields
- Add fields to group

Can change preferences for sites, persons and groups

Close #108

Advances #124

- 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.