Conpherence - fix policy bug on comments with files in rooms

Summary: Fixes T7840. Add some unit tests. One explicitly covers the case in T7840. Other cases are "base" cases for rooms and threads, plus the version if T7840 for a thread, where a policy exception should be thrown.

Test Plan: arc unit and was able to successfully add a file to a conpherence room i did not have edit permissions on

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7840

Differential Revision: https://secure.phabricator.com/D12443