Create Checklist model

- Also added the foreigner gem for FK support in migrations
Add .DS_Store
Add controller + view-related code for Checklists
Update based on PR#9 feedback

- Change set_project to trigger for all actions in ChecklistsController
- Refactor set_project_and_checklist in ChecklistsController
- Add an attr_reader + helper_method for checklists in ChecklistsController
- Add another check to ensure users can get to an edit page from the Checklist index page