Conpherence - add per thread notification setting

Summary: Introduces a new settings panel for Conpherence specific settings.

Test Plan:
started a thread with a test user, thus two participants total. Replied to conpherence, toggling notification settings in between. Verified 1 or 2 emails were sent as appropos to the current toggle.

Toggled global setting and verified setting was updated in conpherences where nothing was specified. Verified setting conpherence setting overrides global setting.

Reviewers: epriestley, chad

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2521

Differential Revision: https://secure.phabricator.com/D5391