Backend: Implement application logging

[flymia]

We need to implement an application log, which logs actions from users and also changes in Eloquent models somewhere.

We also need to implement a LOG_LEVEL setting.

[flymia]

Maybe use something like a trait: https://www.barryodonovan.com/2020/04/27/logging-changes-in-laravel-eloquent or maybe https://github.com/spatie/laravel-activitylog