Move subscription updates to the client layer

This is part of #112 (closed).

I created a separate client for sharing lists of available forums, as that turned out to be cleaner than using the forum client to manage two kinds of group. The same will presumably be true for the blog and group messaging clients when we come to implement them.

The UX for sharing forums is due to change - instead of sharing a list of available forums with no explanation, we'll explicitly invite contacts to join forums. When that happens, the ForumSharingManager will change but the ForumManager should remain pretty much the same.

When hooks for message status changes have been implemented, the ForumSharingManager will use those instead of listening for MessageValidatedEvents.

When client layer transactions have been implemented, the ForumManager, ForumSharingManager and TransportPropertyManager will use them instead of locks to ensure atomicity and isolation.