[config] Make app root URL settable by environment

Developers will often use a different root URL in local environments than in production ones, such as ‘example.app’ or ‘example.dev’. We then need to make this setting modifiable on a per-environment basis.
[config] Update the main configuration file

- Make the `url` setting use an environment variable
- Update the timezone to use Belgian hour
- Remove the default value of the encryption key
[security] Do not expose framework name in cookie

This changes the default name of the session cookie to avoid needlessly exposing the type of framework that is used.

Showing the framework and, by extension, the programming language it’s written in could make the application more vulnerable by suggesting specific targets to potential attackers.