Add a "Thread Members" object policy and some unit tests

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