Add review count to courses

[jameslzhu]

Suggested by Dan Garcia.

Review count matters as much as review rating (since a 4.5 rating by 1000 people is more reliable than a 5 rating by 2 people), so the course professor ratings should reflect this.

[MingweiSamuel]

The right way to do this is to treat it as a statistical distribution and calculate the lower bound (usually 95% confidence) on the true rating and use that stat to sort

[MingweiSamuel]

But I guess this is a UI rather than UX issue