Add optional keystroke support for AphrontPagerView

Summary: This is sort of silly but maybe useful? The real problem is that there
are like 500k conduit call logs and the real solution to that is better
filtering options, but this seems sort of okay.

Test Plan: Used "[" and "]" to switch between pages on the conduit call log.

Reviewers: btrahan, jungejason, nh, aran

Reviewed By: nh

CC: aran, nh, epriestley

Differential Revision: 1145