Give Pholio Images an authorPHID and use ExtendedPolicies to implement policy behavior

Summary:
Depends on D19912. Ref T11351. Images currently use `getMock()->getPolicy()` stuff to define policies. This causes bugs with object policies like "Subscribers", since the policy engine tries to evaluate the subscribers //for the image// when the intent is to evaluate the subscribers for the mock.

Move this to ExtendedPolicies to fix the behavior, and give Images sensible policy behavior when they aren't attached to a mock (specifically: only the user who created the image can see it).

Test Plan: Applied migrations, created and edited mocks and images without anything blowing up. Set mock visibility to "Subscribers", everything worked great.

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T11351

Differential Revision: https://secure.phabricator.com/D19913

resources/sql/autopatches/20181218.pholio.01.imageauthor.sql

@@ -0,0 +1,2 @@
+ALTER TABLE {$NAMESPACE}_pholio.pholio_image
+  ADD authorPHID VARBINARY(64) NOT NULL;