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
Break before_action :set_status_item into memoizing reader methods and move view-specific methods to
a helper

Thanks @zporter
Remove `array_of_` from the #create_status_items_for input variable name
Remove unnecessary whitespace
Use memoized helper methods
Remove dependent: :destroy from :status_category association
Re-render on #create and #update instead of redirect when errors are encountered