Mark ChatLog as deprecated

Summary: Ref T6875. #chatlog is deprecated and will eventually be merged into #conpherence.

Test Plan: N/A

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: cburroughs, epriestley

Maniphest Tasks: T6875

Differential Revision: https://secure.phabricator.com/D12097