Use 'forum' instead of 'group' where appropriate. #174

With the separation of the block sync protocol from its clients (messaging, forums and transport properties), the concept of 'group' is being split into 'channel' (which exists at the block sync layer) and client-specific objects like 'forum' (which exist at the client layer).

This patch renames a load of stuff in the forum UI. This is the first part of #174 (closed).