Story 768: Add previews to editor

* Add a preview button in between previous and next buttons
* Add a close button on the preview screen in same position to make it easy
to escape
* In preview window load a preview of article using Section view (now
exposed via define )
* Fix the overlay header to be aligned with the back arrow
* Adds edit-preview event, make use in toggle and cleanup templates module to add
this to preview

Change-Id: I1f4bee08cf24c2ce2657d46b650b3e1a0532d408