Allow "J" and "K" to jump between files in Differential

Summary: Provide a more coarse keyboard navigation option to jump between files.
Test Plan:

• Used "j" and "k" to jump between changes in files.
• Used "J" and "K" to jump between files.
• Pressed "?" and read help about this.

Reviewed By: jungejason
Reviewers: jungejason, tuomaspelkonen, aran
Commenters: fzamore
CC: aran, epriestley, jungejason, fzamore
Differential Revision: 764