Add Hint to each hint

Summary:
There currently is a "Hints" title over all the hints, and this changes it to have a "Hint" title over each hint. There are several reasons for this.
(1) Some hints are really long, and it's not clear what they are.
(1x) "but of course it's clear what they are, Brian, they're hints; everything after a certain point are hints, duh"
(1y) yea "1x", but if I integrate widgets into hints, then everything is interspersed and it's terrrible. With three 'r's
(1z) Did you notice that Hint is really small and without a ":""
(1y) Stop with the details, this is one in a chain of diffs that just helps you see the logical flow/evolution of changes.

Reviewers: jack

Reviewed By: jack

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D6917