Modify application settings in the web UI

[aronmolnar]

Application-wide settings are currently configured via the config file app.env (https://docs.sysreptor.com/setup/configuration/).

It is much more user-friendly if those settings could be modified via the web UI. This doesn't make sense for all configuration options (like the Django secret key, encryption keys, and more). But all options that are feasible to be set via the web UI should be editable there.

This is also valid for plugin settings. Plugin developers could create a schema, which configuration options are available, which could then be presented via the web UI.