Phortune v0

Summary:
Ref T2787. This does very little so far, but makes inroads on accounts and billing. This is mostly just modeled on what Stripe looks like. The objects are:

  - **Account**: Has one or more authorized users, who can make manage the account. An example might be "Phacility", and the three of us would be able to manage it. A user may be associated with more than one account (e.g., a corporate account and a personal account) but the UI tries to simplify the common case of a single account.
  - **Payment Method**: Something we can get sweet sweet money from; for now, a credit card registered with Stripe. Payment methods are associated with an account.
  - **Product**: A good (one time charge) or service (recurring charge). This might be "t-shirt" or "enterprise plan" or "hourly support" or whatever else.
  - **Purchase**: Represents a user purchasing a Product for an Account, using a Payment Method. e.g., you bought a shirt, or started a plan, or purchased support.
  - **Charge**: Actual charges against payment methods. A Purchase can create more than one charge if it's a plan, or if the first charge fails and we re-bill.

This doesn't fully account for stuff like coupons/discounts yet but they should fit into the model without any issues.

This only implements `Account`, and that only partially.

Test Plan: {F37531}

Reviewers: chad, btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2787

Differential Revision: https://secure.phabricator.com/D5435