add update to rental requests

 - add buttons to CatsController#show to update request status
 - add CatRentalRequest#approve to approve request
 - update logic of rental overlap validation so that only approved rentals are counted
remove unneeded rental routes

 - no edit form--this happens from CatsController#show
 - no delete (dependent destroy)
add users and authentication

 - add user model/controller
 - add sessions model/controller
 - add helper methods to track authentication
fix authorization/authentication

 - cats now autopopulate owner id based on current user at time of creation
 - users can now only make decisions about rental requests for the cats that they own
 - content is only available to logged in users.
fix rental controller logic; refactor rental model

 - Rental controller now appropriately cancels requests that interfere with an approved one.
 - Fixed CatRentalRequest overlap check to use clearer helper methods.