Implement preview for topic replies
Restore message when not logged in
Add PreviewsController

Replace RepliesController#preview with PreviewsController in order to support replies for all contribution derivatives.
Extend preview functionality to comments
Remove contribution_body partial
Remove not needed include
Minor improvement in routes
Implement throttled preview with underscore
Add preview to announcements and topics
Add preview functionality in message_boards
Move reply preview bellow the reply box
Modified contribution preview box styles
Fix code style in reply preview
Fix styles in preview when editing a reply
Refactor topic_controller_spec
Fix identation problems
Fix naming in message_boards.coffee
Refactor replies edit view
Minor change in reply preview style
Make preview box disappear if there is no content