R1:a84cd9989398

Paste: Add edit policy

Summary: T5549

Test Plan: Set edit policy on paste, check that only users meeting the policy requirements can edit it.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T5549

Differential Revision: https://secure.phabricator.com/D11097