Summary:
Ref T8488. Ref T5681.
Now you can just use `id(new ConpherenceThreadMembersPolicyRule())->getObjectPolicyFullKey()` as a policy.
Added tests for TaskAuthor + ThreadMembers.
Test Plan:
- Ran tests.
- Set a thread policy to "Members of thread'.
Reviewers: btrahan
Reviewed By: btrahan
Subscribers: epriestley
Maniphest Tasks: T5681, T8488
Differential Revision: https://secure.phabricator.com/D13258
