Add file web links for preview in side-by-side diff view

In the side-by-side diff view there is now a preview icon to render
Markdown files as HTML. This way the formatting can be easily checked
while reviewing.

Change-Id: I24bffa1ebdab31bd18c965d9b76c0f5b1a9ec509
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>