R1:868ff166b109

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…