Make static pages use ContentGroup.groupinfo_by_id
Correctly show parent when editing exam

* Correctly display the parent on the exam edit dialog for child exams.
  (closes #1825)
  (closes #1914)
Fix return type of create_exam's prefill_children

* Addressing comments on PR #1918, fixing the return type of
  prefill_children and it's call signature in more places.
* Also cleans up some validation errors in create_exam.html