Give Phortune merchants explicit members

Summary:
Ref T2787. Make this a little more concrete with explicit membership instead of a general edit policy. In particular, we need to know who to email when orders happen, and can't reasonably do that with an edit policy.

I imagine this might eventually get more nuanced (e.g., users who can only approve orders vs users who can manage the merchant itself) but that's a long ways away.

Test Plan:

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T2787

Differential Revision: https://secure.phabricator.com/D10681