Fix multiple references to the same file in Xcode.

Signed-off-by: Aron Cedercrantz <aron@cedercrantz.se>
Fix crash when OPFQuestionsViewController was deallocated.

- We did not stop observing changes for `sortCriterion` and `sortORder`.

Signed-off-by: Aron Cedercrantz <aron@cedercrantz.se>