Some AppContext tests fails

[Lenkelheit]

The tests with checking the cascade deleting(1) and some entities null exceptions(2) fails. It happens because 1 - all logic that replaces the cascade deleting is in repositories' classes, so it only normally works with repositories, but not AppContext. In future we can rework it, for example, to triggers or to smth else. 2 - for good work with data almost all relationships between entities are setted to HasOptional from HasRequired, so some entities allow to have null value.

Reference to commit - 50b351699f186747a1029ab639702005408a519f

[iamprovidence]

We have some ideas to fix this in #59 . Maybe we should try something with triggers