Users can create and view listings

* Generate migration for listings
* Create Listing model with relationship to User
* Add validations to Listing model
* Add has_many relationship to User model
* Add monban constraints and root to routes
* Add listings_controller with new, create, and show actions
* Add views for listings#new and listings#show
Clean Up Layout

* Fix incorrect route to sessions#new
* Add Home, Sign Out, Sign In, and Sign Up links to application layout
* Add h1 heading to application layout
* Add h2 headings to other views