Status Categories and Status Items (#29)

This PR introduces two new concepts, a `StatusCategory` and a `StatusItem`.  Here's a breakdown of what this PR includes:
- `StatusCategories` are polymorphic as `categorizable` so other things can be assigned categories down the road
- `Checklists` are the first thing to be categorizable
- `Status Items` are polymorphic as `statusable` so other things can be assigned status items down the road
- `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` (having the items display their status as either a `boolean` or `string`)

I went for polymorphism because I have a ticket in the pipeline for "Test Plans" in which I'll be able to use the `StatusCategory`/`StatusItem` functionality.