Add data integrity checks

How to reproduce this issue?

The obvious way: for example save a Course with empty lessons.

We should be able to use entity constraints for this.

@catch, we can still implement some constraints - these will prevent edition of an entity. So far I prevented deletion and added some warnings on edition attempts. Pls check when you'll have some time.
I may add some automated tests later.

Thanks for reviewing!

Automatically closed - issue fixed for 2 weeks with no activity.