R1:9404e2b3d425

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.