Improvements to downloading file submissions in SAMigo

The improvements include:
-making it easier to find bulk download function on the Questions page (plus a few other layout/label improvements to the page)
-adding ability to select multiple sections when bulk downloading student file (new Download File Submissions page)
-some UI tweak in all the grading pages (please see UI Improvement of Score pages.doc)

Note: The spec is first made for Stanford (CW-3109 and CW-3146). The word "assignment" will be changed to assessment for Sakai.