Private Group Threaded Conversation

This MR refactors the forum activity, its controller, its adapter and view holder so *most* of the code can be re-used for private groups by making heavy use of generics.

The refactoring has 1383 additions and 1087 deletions, so just grows the code-base slightly and adding the private group conversation just takes an additional 400 lines.

The MR also includes one commit that moves post/message creation more into clients, so the UI doesn't need to keep track of timestamps. This commit can of course be split out into a separate MR if desired.

Closes #662, #663 

See merge request !350