Add more keyboard navigation options for inline comments

Summary:

• Use n/p to jump between comments.
• Use r to reply to the selected comment.
• Use e to edit the selected comment.

Test Plan: Verified n, p, r, e, j, k, J, K, "click edit" and "click reply"
behavior in as many weird cases as I could come up with.

Reviewers: btrahan, jungejason, nh, cpiro, jl

Reviewed By: btrahan

CC: aran, btrahan, epriestley

Maniphest Tasks: T583

Differential Revision: https://secure.phabricator.com/D1308