no dots in html dom ids else jquery lookup fails

The staff grading-submission modal window will fail to appear if the course name or location name has a period in it.  This is because jquery interprets such id's as a js element lookup (see http://stackoverflow.com/questions/17403636/jquery-x-does-not-work-but-document-getelementbyidx-works).

This PR fixes the issue by replacing "." with "_" in the Context id string.