R1:2d0ee77bd4fb

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: {F216284}

Reviewers: btrahan

Reviewed By: btrahan

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