OO-1038: better check of long without exception, try and catch in PersistingCourseGroupManager, unit tests

-no-jira: replace escaping with antisamy for group description as the description is edited with TinyMCE (which already escape special characters)

OO-716: replace the flot library in the course statistics with the bar chart component which use d3.js

OO-1014: give the right name to identify the type of image in the repository details, fix the "grayed tone all pictures"!