Assign topic/assoc to another workspace
1st minimal concept:
- workspace assignment is editable via "Meta" tab (as this is where the current assignment is displayed)
- as a consequence: changing workspace assignment works only for a single topic/assoc
Access control:
- user needs WRITE permission for both, the topic/assoc, and the target workspace.
This is a follow-up of dmx-intern/sprint-planning#75