Enhanced PholioMockQuery

Summary:
Images and cover files can now be attached calling need functions for PholioMockQuery.
Mock list will show cover files for mocks.
MockView uses new feature to show the first image

Test Plan: Verified that images are shown.

Reviewers: epriestley

CC: aran, Korvin

Maniphest Tasks: T2364

Differential Revision: https://secure.phabricator.com/D4644