#738 - Roll basic inventory into the framework
Encapsulated changes in this commit:
- Remove outdated Sku availability services
- Add new inventory services
- Add new activities to check availability and decrement inventory (not hooked up by default)
- Added a weave class transformer for those that are on 3.1 and still want to use inventory
- Add an extension manager to handle 3rd-party inventory services
- Add new InventoryTypes and decrement old ones
- Deprecate the old 'available' flag
- Hide deprecated InventoryType values in the admin
Fixes #739 - Add some NPE protection to DirectCopyClassTransformer