Add ability to render text as markdown (#28)

- Implemented with Redcarpet gem
- Render View#note as markdown
Move #markdown from ApplicationController to ApplicationHelper
Styles where we landed field
Create Checklist model

- Also added the foreigner gem for FK support in migrations
Add controller + view-related code for Checklists
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
Remove dependent: :destroy from :status_category association
Add TestPlan model and extract out shared functionality w/ Checklist into a ProjectViewList concern
Wire up controllers + view code for Test Plans, DRY up ProjectViewList-related code (Checklists + Test Plans)
Create TestItem model and add necessary associations
Implement TestPlans and TestItems (#30)

- Also re-structure some controllers/views/routes