fixed a bug at day-queries-service

I missed a replacement mishap in !239 and fixed it :D

But then I stumbled across another bug, closely related: Previously we referenced the defaultSchedule in QuestionRelations (formerly known as QuestionSchedules), where the QuestionRelation had no specific question of its own. The reason for that was, that when editing the schedules we accidentally edited the default Schedule. Now the DayQueriesService failed to find the appropriate Schedule, because it was looking at the QuestionRelations and found undefined. Therefore I introduced SymptomCheck.getEffectiveSchedule() that returns either the specific Schedule for a question or if need be a copy of the default schedule. That's it :)

@CRSchmidt can you take a look?

Closes #372 (closed)