Permission managment for adding playlist entries of type "Stream"

Parent: [EPIC] Minimal role and permission management (aura#156)

To avoid the possible issue of constant licensing problems or RMs avoding to produce original content it could be helpful to implement following:

• By default only allow adding streams to playlists by Programme Coordinators
• Other users might be able to do that too, if they got the releveant permission assigned (via Steering)