Fix simultaneously running contribution and interposed question

[Elblinator]

Current behaviour:
An interposed question and it's parent can run at the same time

Reproduction:

• activate interposed question
• navigate to a LoS
• Put someone on the LoS and start their contribution
• put two interposed questions on the LoS
• start the interposed question (the parent automatically stopped)
• start the other interposed question (both questions continue)
• start the parent (the interposed questions continue)

Wanted behaviour:
If the parent is started the interposed questions should automatically be stopped and if the interposed question is started the parent/other question should automatically stopp.
The contribution which is started last should run and automatically stop the other ones