Give Pholio mocks a configurable edit policy

Summary:
Ref T4566. Currently, mocks have a conservative (author only), immutable default edit policy.

Instead:

  - Let the edit policy be changed.
  - Default the edit policy to "all users", similar to other applications.
  - Add an application-level setting for it.
  - Migrate existing edit policies to be consistent with the old policy (just the author).

This stops short of adding a separate "owner" and letting that be changed, since Pholio doesn't really have any review/approve type features (at least, so far). We can look at doing this if we get more feedback about it, or if we make owners more meaningful (e.g., add more "review-like" process to mocks).

Test Plan:
  - Ran migration scripts.
  - Confirmed existing mocks retained their effective policies (author only).
  - Created a new mock, saw edit policy.
  - Changed edit policy.
  - Changed global edit policy default.
  - Tried to edit a mock I couldn't edit.

Reviewers: chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T4566

Differential Revision: https://secure.phabricator.com/D9550