Fix bad "editPolicy" key in Paste

Summary: Fixes T12508. Files don't have an editPolicy, and we started actually checking that the keys are real things in D17616.

Test Plan:

• Before patch: created a paste, got an "editPolicy" exception.
• After patch: created a paste that worked properly.

Reviewers: avivey, chad

Reviewed By: avivey

Maniphest Tasks: T12508

Differential Revision: https://secure.phabricator.com/D17628