Remove docextra for Forum entities

Currently we have

#set($showcomments = false)

for TopicSheet and ForumSheet.

The logic behind this IMO is that since this application is focused on comments, the default comments functionality overlaps its purpose.

I suggest to remove also the other #docextra functionality since it's crowding the interface and they are not used in the context of the application.

If someone want to add an attachment to an answer he can use the WYSIWYG editor. The attachment won't be visible in the Attachment's tab since answers are generated and hidden (example of answer URL Topic#.U3xhpFiSyLN )

The history and informations tabs can be accessed through ?viewer= functionality.