Better course page statistics

[bmesuere]

Right now, on the top of the course page, we have a few basic statistics (users, exercises, submissions) and a link to the dedicated statistics pages.

These values are not super useful in a "live" course. The statistics on the top of the page should focus on what is happening right now (with live updates?).

Some ideas:

• Number of students online (members with a page view in the last x minutes) Note that this might not be 100% correct if a users has multiple Dodona courses at the same time.
• Number of submissions in the past hour: could be a bar chart with one bar per minute
• Exercise/series being worked on right now

### Related issues
- [ ] #958 
- [ ] #987