Implement deletion concept for RfCs

[MrSerth]

So far, RfCs cannot be deleted (nor edited) by anyone yet (except for removing the question text in case of an explicit question). We should consider allowing an update to the RfC (like the question for learners) or even "deleting" the RfC in case it was created by accident. Probably, soft-deletion would be the way to choose, so that we don't loose the data immediately.

Some teachers have also raised the need to "hide" some RfCs from fellow learners in the past (especially in an in-class setting). However, this is probably another feature than the deletion of RfCs.