Quiz reporting does not scale to a large number of user enrolments

get_users_by_capability() without any limits is used throughout the quiz reporting, in courses with many enrolments this results in the max_memory_limit being hit and very large queries being created because of the use on IN() statements filled with enrolled users id's.

Replacing get_users_by_capability() with get_enrolled_sql() may be a better approach. In my own testing it performs much better, but there may be some corner cases I'm not aware of that mean that get_enrolled_sql may be appropriate.