Implemented Mentionable interface in Pholio

Summary: closes T8486

Test Plan: Create a pholio mock. In an already existing object mention the pholio mock. Verify that the reference to the object exists in the pholio mock's timeline.

Reviewers: lpriestley, Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T8486

Differential Revision: https://secure.phabricator.com/D13402