Delete SectionEvent with related entities

When SectionEvent is deleted and there are ImageRef entities, if fails with foreign key constraint failure.

AC

• on deleting SectionEvent, make sure ImageRefs are also deleted
• on deleting Observation, make sure ImageRefs are also deleted
• on deleting ImageRef, make sure ThumbnailRefs are deleted
• check the possibility to delete the images also in the bucket
• tests are running

Example request:

mutation {
    deleteSectionEvent(
        input: {
            id: "/tmd/api/index.php/section_events/33"
        }
    )
        {
            sectionEvent {
                id
            }
        }
}

Error message:

"debugMessage": "An exception occurred while executing 'DELETE FROM \"observation\" WHERE id = ?' with params [47]:\n\nSQLSTATE[23503]: Foreign key violation: 7 ERROR:  update or delete on table \"observation\" violates foreign key constraint \"fk_4c85eef01409dd88\" on table \"image_ref\"\nDETAIL:  Key (id)=(47) is still referenced from table \"image_ref\"."