Make ItemRenderer hints controlled
Make ItemRenderer hints controlled

Summary:
The new exercise framework keeps track of how many hints are
currently shown, so this removes the funky `initialHintsVisible` prop
and the `.showHint()` function in favor of a single `hintsVisible` prop.

This is a pretty small change, but I mostly am curious if anyone else is
using/planning on using `ServerItemRenderer`, and if so, do they want me
to keep the original `initialHintsVisible` behaviour?

Test Plan:
 - Load an exercise in the new exercise framework
 - See that there are no errors and hints show up

Reviewers: samiskin, charlie, jared

Reviewed By: charlie, jared

Differential Revision: https://phabricator.khanacademy.org/D28354

ariabuckles/perseus,ariabuckles/perseus,learningequality/perseus,ariabuckles/perseus,ariabuckles/perseus,learningequality/perseus