Implement PolicyInterface, ExtendedPolicyInterface, and DestructibleInterface on PhrictionContent

Summary:
Depends on D19093. Ref T13077. Although content objects normally don't have any edges today, they may in the future.

Also implement Policy stuff properly.

Test Plan: Used bin/remove destroy to destroy a document, verified it also loaded and destroyed the correspoding Content correctly by looking at --trace and the database rows.

Maniphest Tasks: T13077

Differential Revision: https://secure.phabricator.com/D19094