Show prices in "$... USD" in Phortune

Summary:
Ref T2787. See discussion in D5834.

  - Replace `PhortuneUtil` with `PhortuneCurrency`, which feels a little better and more flexible / future-proof.
  - Add unit tests.
  - Display prices explicitly as "$... USD".

Test Plan: Hit product list, cart, purchase flow, verified displays.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2787

Differential Revision: https://secure.phabricator.com/D5841