Simplify comment highlight behavior in Pholio

Summary:

• Currently, we register this behavior every time we load comments. Instead, register it once.
• We have two very similar behaviors for over/out. Just use one that does the right thing based on the event type.

Test Plan: Waved my mouse over areas of the image with reticles, saw the correct comments highlight.

Reviewers: chad, ljalonen

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D5082