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