Remove Round remove CandidateList

When removing a Round should also delete all the CandidateLists linked.

changed milestone to %Candidates

added Backend label

assigned to @apoyen

added To Do label

added Doing label and removed To Do label

created branch 26-remove-round-remove-candidatelist to address this issue

created merge request !33 (closed) to address this issue

mentioned in merge request !33 (closed)

mentioned in merge request !34 (merged)

closed via merge request !34 (merged)

mentioned in commit a49cfa61