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