Delete a Desk should delete all DeskRound linked

Modify Desk DELETE API to delete all DeskRounds in cascade.

changed milestone to %Round election management

added Backend label

assigned to @apoyen

added To Do label

changed due date to May 29, 2020

added Doing label and removed To Do label

created branch 21-delete-a-desk-should-delete-all-deskround-linked to address this issue

mentioned in merge request !19 (merged)

closed via merge request !19 (merged)

mentioned in commit e6e6952a