- 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.

WTF poetry?!

https://github.com/django-dbbackup/django-dbbackup/issues/314

This reverts merge request !61

 * Update django-two-factor-auth to current release
 * Allow phunnumber-field 3.x or 4.x while django-two-factor-auth
   still crunch https://github.com/Bouke/django-two-factor-auth/pull/326
 * Run poetry update
   * Update django-image-cropping and easy_thumbnails for DJango 3.0 support
     (cf. !61 and #75)