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
