Add ability to not back up inactive courses

There should be 2 additional parameters in the automatic course backup settings page.

The first one should ask which courses to backup: visible, non-visible(might be useful for archiving) or both

The second one should ask for how long a course should be backed up after the last modification. It is actually hardcoded to 31 days, which seems to be quite arbitrary. Knowing that the backup process is veeeery slow, it could help to lower that number (especially after importing over 1000 empty courses at the beginning of the year).

IMO, this should be fairly easy to implement

Thanks in advance!