add dynamic_settings

[felixrindt]

The goal is that at least some settings used after project initialization can be customized by plugin code.
I though about replacing the django.conf.settings object we use everywhere, but that caused circular-import-hell. Now it's very explicit when the dynamic mechanism is used. I like that we can use any prefix or other mechanism to provide defaults in settings.py using defaults from the env-file (feel free to suggests other names/mechanisms).

TODO

• add dynamic settings object
• migrate SITE_URL and USERCONTENT_URL
• migrate brand_logo signal to brand_static_path
• add platform name and primary color

[coveralls]

coverage: 83.904% (+0.03%) from 83.874%
when pulling 18d50f5 on dynamic-settings
into 8707358 on main.