R1:ff4fa1885b4e

Improve pagination in ChatLog application

Summary:
- Add an extra paginator at the top.
- Add a link to jump to the bottom (where the latest messages are).
- Align paginators with edge of content rather than the page.

Test Plan: Looked at the chatlog.

Reviewers: epriestley, chad, #blessed_reviewers

CC: chad, Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D7280