Simplify the management of the forum entities

The ForumCode.Macros document has some macros that are doing the same thing for different entities (a lot of duplicated code: #displayTopicAddForm($space) and #displayConversationAddForm()). This can be done in a single generic macro.

Another thing to be investigated is the way the comments are added/displayed (most of the code is copied from the commentsinline.vm) and this could be simplified by including the commentsinline.vm template (but this is available only in flamingo skin) and overriding only the needed macros. Some macros that are fully copied from commentsinline.vm:
#addCommentToThreads($comment, $commentThreads, $rootKey), #displayThread($rootKey, $commentThreads, $commentDoc).