Adding an activity with a grade item previously triggered a course
regrade, which can be extremely slow. This change makes it so that on
courses where regrading is slow, when you add an activity that
requires regrading, it takes you to an intermediate screen with a
progress bar.
This change also fixes a bug in the existing grade overview report,
which shows grades across multiple courses, but previously only
checked, and if necessary regraded, one course.
