R1:47b54389e5a3

Forbid adding non-users to Conpherence threads

Summary: Fixes T6724. Adds validation that participants are users.

Test Plan:
- Tried to add non-users, got an error.
- Added users normally.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6724

Differential Revision: https://secure.phabricator.com/D11955