Redesign configuration

Weblate uses Django settings since the beginning. It might be time to revisit this decision as in many cases this causes troubles.

- The settings is Python code, what makes it powerful, but most people do not need this
- The settings live in the Python site packages, what makes it look like code, while it actually is not
- Most of the settings are not supposed be changed by users and forgetting doing that on upgrade can lead to subtle breakage
- We have additional layer on top of that to load settings from the environment in Docker container, maybe we could use something shared for both environments
- The settings lack type annotations and checking, which makes it easy to break.