Fix #556, use relative URL for css background image

If use absolute URL, it will cause problem when the site is not on root
of the web server, e.g. image missing
Fix #554, confirm button on upgrade page on firefox
Fix #555, merge user search failed when installed on subdirectory
Refactor rubric form to show header in view page
Refactor mixeval to use unified preview

The preview section shows the same as what student sees