WIP : CFD-144 log group activity

Modules to support logging activity in the message table for:
* create/edit/delete node content.
* create/edit/delete comments.
* activities about users and group membership.
matter if taste, but I prefer to save one line and do the assignment inside the if

``if (!$config = variable_get(c4m_activity_comment_variable_name($type), FALSE)) {``