Delete connected entities

[TheSlimvReal]

We provide the functionality to delete entities like children or schools.
However, we did not implement anything to delete or alter connected entities.

For example if a Child is deleted, there are still ChildSchoolRelations that are connected with this child which are not deleted.
This can cause problems in this case in the SchoolDetails component when all children attending this school are loaded.

We should think of a concept which also handles connected entities. One approach would be to delete them as well.

[sleidig]

Duplicate of #220 - for things like ChildSchoolRelation there is definitely a need for this, though.