Model-level implementation of StatusCategory and StatusItem

- StatusCategories are polymorphic as categorizable
- Checklists are the first thing to be categorizable
- StatusItems are polymorphic as statusable
- Views are the first thing to be statusable
- The status of a StatusItem can be represented as either boolean or string
- StatusCategories can toggle the status type for all associated StatusItems
Wire up controller and view code for StatusCategories and StatusItems