New ForumSharingManager based on ProtocolEngine

In order to improve the UX for sharing forums, the ForumSharingManager needs to be rewritten. Similar to the IntroductionManager, it should make use of the abstract Protocol engine and implement a protocol (#320 (closed)) between two parties.

This is a subtask of Issue #121 (closed) which concerns the UX. The interface with the UI is discussed in #321 (closed).