CSS polish, Pholio

Summary:
Minor things

- Fades out comment icon on hover
- Adds hover to inline comment images
- moves mask position to just the image, and not the transparent border

Test Plan: Tested all of these items on various mocks

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9631