Cache result from `get_courses_average_grade`

[pkevan]

On large sites, the query resulting from get_courses_average_grade can result in excessive query time.

Caching the result of this query for 1hr (or longer) should be sufficient in reducing excessive load without necessarily reducing accuracy. A recalculation or removal of the cache value after completion of a course could be possible.

Example: https://learn.wordpress.org/course/beginner-wordpress-user/