Connect working days administration page and background job

[ulferts]

https://community.openproject.org/wp/44214

TODO

• [x] Trigger WorkPackages::ApplyWorkingDaysChangeJob whenever the working_days Setting is altered.
• [x] Avoid having working_days Setting specific code in the Settings::UpdateService
  • A on_change callback parameter is introduced that gets called by the Settings::UpdateService whenever the setting is altered. That way, every definition can define an on_change parameter.
• [x] Add a spec for Settings::UpdateService