Add 'Persist Chat' option in Conpherence notification menu

Summary: This exposes the chat window to a larger audience beside people who accidentaly hit \.

Test Plan:
Lots of clicks and reloads.

{F1856043}

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D16643