Closing a course does not remove participants, editor not blocked

When closing a course, the code suggests the participants would be removed, however this was not the case when a client accidentally closed a course.

The code in CloseResourceCallback needs to be checked:

• is doCleanGroups really called?
• change code to only remove group members if the group as no other courses references
• if a group as other course reference, remove the course reference instead of removing the users from the group
• implement the empty method RepositoryServiceImpl.removeMembers() to remove course members when closing a course
• add administrative logging to the course log file about who triggered to course close action
• prevent opening the course editor when course is closed