Fix a Phortune bug where an invalid viewer could sometimes be selected for billing a subscription

Summary:
A live instance hit the scenario described in the comment, where an out-of-date user was being selected as the actor.

Since they were no longer an account member, they could not see the payment method and autopay was failing.

Instead, select a relatively arbitrary user who is a current, valid, non-disabled member.

Test Plan: Ran subscriptions with bin/worker execute ..., saw it select a valid actor.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D16757