switch to pydantic in settings management

[adamjanovsky]

Management of global configuration variable is now left to Pydantic. This means:

• No json schema, this is replaced by Pydantic specification
• Description of default values and types is now accessible from docs, not from the example yaml
  • The docs no longer show the configuration keys twice 🙃
• No default settings.yml is needed anymore, the default values are stored in the class
• One can define new settings files just by overriding some keys, see test settings e.g. where only a single key is overriden.
• One can set keys with environment variables -- handy for secerts
• Some old keys got deleted, some sensible integer boundaries on existing keys were introduced
• I wrote some tests to check that new config works properly

[adamjanovsky]

@J08nY could you please briefly check this? If you build docs, the page configuration.html contains more elaborated description on how to work with the configuration, but little has changed.

I say we don't add support for loading secrets from files. Environment variables should be ok for now, right?