Make chatlog a bit less awful

Summary:

• Default to showing the newest page of chat.
• Reformat for greater readability.
• Add permalinks to specific lines.
• Enable jump-to-date.

Test Plan: {F12200}

Reviewers: Koolvin, vrana, btrahan

Reviewed By: btrahan

CC: kdeggelman, aran

Maniphest Tasks: T837, T1065

Differential Revision: https://secure.phabricator.com/D2641