Chatlog - add a pager

Summary: 'cuz I miss out on chat room goodness and can't paginate around in the current version

Test Plan: setup a phabot and spammed it in phabot-test. with new test data, set $page_limit = 1 and paged about -- looks good!

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Maniphest Tasks: T990

Differential Revision: https://secure.phabricator.com/D2032