Display questions belonging to compared scenarios.

Before, when you ran `analysis`, the results page would loop through
the questions of the `question_set` of the current `Partition`.

However, when you want to compare old scenarios that have (currently)
disabled questions, this does not work.

Please take into account that all the scenarios should have been made
with the **same** questions!
Merge pull request #232 from quintel/compare

Display questions belonging to compared scenarios.