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
Move Phortune product logic into applications

Summary: Ref T2787. `Product` is currently a fairly heavy object, but as Phortune develops it makes a lot of sense to make it a lighter object and put more product logic in applications. Convert it into a fairly lightweight reference to applications. The idea is that Phortune is mostly providing a cart flow, and applications manage the details of products.

Test Plan: Funded an initiative for $1.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T2787

Differential Revision: https://secure.phabricator.com/D10634
Add more structure to Phortune product purchasing flow

Summary:
Ref T2787. When a user purchases a product in Phortune, transition the cart through a purchased state and invoke product callbacks so applications can respond to the workflow.

Also shore up some stuff like preventing negative amounts of funding.

Test Plan: Backed an initiative and saw it show up on the initiative after completing the purcahsing workflow.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T2787

Differential Revision: https://secure.phabricator.com/D10635
Fix incorrect maniphest.update conduit UI

Summary: Fixes T6254 and renames status as string. Though maybe this should go through `formatStringConstants`?

Test Plan: Reload Conduit page, see new text.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6254

Differential Revision: https://secure.phabricator.com/D10637
Merge branch 'master' of github.com:facebook/phabricator