Make Currency a more formal type

Summary:
Ref T2787. Phortune currently stores a bunch of stuff as ...inUSDCents. This ends up being pretty cumbersome and I worry it will create a huge headache down the road (and possibly not that far off if we do Coinbase/Bitcoin soon). Even now, it's more of a pain than I figured it would be.

Instead:

• Provide an application-level serialization mechanism.
• Provide currency serialization.
• Store currency in an abstract way (currently, as "1.23 USD") that can handle currencies in the future.
• Change all ...inUSDCents to ..asCurrency.
• This generally simplifies all the application code.
• Also remove some columns which don't make sense or don't make sense anymore. Notably, Product is going to get more abstract and mostly be provided by applications.

Test Plan:

• Created a new product.
• Purchased a product.
• Backed an initiative.
• Ran unit tests.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T2787

Differential Revision: https://secure.phabricator.com/D10633