[C2] Include optional tracking of users of a course who are not subscribed

[ywarnier]

In the general (first) course reporting page, you can see results of users that are subscribed to the course.
If a course is "open" but users have entered without registering as students to the course, they are generating data in the tracking tables (database), but will not appear on the reporting pages because it first checks users registered in the course.

Add a button to the reporting page "Show users not registered". When clicking that button, the query changes to include all results (from registered and non-registered users) on the reporting page.

[christianbeeznest]

It is implemented with this commit fcef975

Thanks for confirmation.