As a developer, I want to limit restaurants to advertise only 3 dishes.

Decription
Patat does not specify how many dishes can be advertised. This way, restaurants have the ability to mark every item of their menu as being on offer.

Checklist
- [x] Check how many dishes are advertised already before edit view is opened
- [x] Disable checkbox in case three dishes are already advertise and display some information on that
- [x] Refactore code in Menu.php, so that it is more obvious which code is executed when

@Guido46 Task is ready for review