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.

This finalises the migration to the black code style by applying double-quotes.

https://edugit.org/BiscuIT/BiscuIT-ng/-/jobs/657

It was replaced by the Yarn collector, which can also collect from Debian's
javascript-common tree.

Rationale: Adding logic to decide on the login view to use, and
reliably disable the default login view if 2fa is enabled, adds
unnecessary complexity, compared to django-two-factor-auth being
a quite lightweight dependency.