Make it possible for applications to generate concrete subscriptions

Summary:
Ref T6881. This still doesn't "work" in any reasonable sense of the word, but gets us a bit further.

I'll build out the Phortune UI a little bit next, then look at implementing the Worker to do actual billing.

Test Plan:

• Allocated an instance and saw a Subscription generate properly.
• Saw subscription show up in the Phortune UI, albeit in a very limited way.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6881

Differential Revision: https://secure.phabricator.com/D11575