Add keyboard shortcuts to Pholio

Summary: Fixes T2657. Adds j/k for previous/next image. This is consistent with Differential. We could add left/right later but it needs a little work to make those available.

Test Plan: Mashed j/k, saw images switch.

Reviewers: chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T2657

Differential Revision: https://secure.phabricator.com/D5220