Feature/teletunes questions

[BeritJanssen]

I thought that assigning questions through the admin would work for all experiments now, but it turns out it only works for those which use the experiment.rules.base.get_questionnaire function. Most Hooked experiments get a single question after each round, and use the experiment.rules.base.get_single_question function instead. Since we don't want to fully randomize the questions here, but first get demographic, then musicgens questions, I reasoned it would be faster for now to hard-code the questions "old style", and add an issue for the get_single_question function ( #679 ), as we still need to figure a way to do "randomization in blocks" through the admin interface also.

The branch also adds a test for the playhead shift in song_sync rounds, checking that indeed, the playhead advances by silence_time if the track should continue where it left off, and otherwise silcence_time + a random jitter between min and max values.

[drikusroor]

LGTM. And yes, let's hard-code for now and fix #679 later. This one is supposed to go live soon right?