MDL-29794 Initial support for re-using a shared grading form

MDL-29794 Initial support for re-using a shared grading form

The patch introduces a new script templates.php that allows to search
for a previously shared form (template) and re-use it.

The patch also modifies the preview rendering of grading forms. Now
plugins are responsible for rendering the form itselft, without any
headers, descriptions etc (we need to embed the form preview into
various places so the caller looks after the frame).