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