Adds Session, User, and logins.

ActiveRecord::Base includes SessionHelper to get current user.
Every page displays a login form if no user is logged in, or a
logout form if user is logged in.
Changes page header to show all flash notices.

Removes login header from signup page.
Adds authentication to approve/deny of CatRentalRequests.

And to edit of cats.
Forms reload themselves on failure.

Prefills forms with data from previous failed attempts.
Adds back buttons.
Adds CSS PINK
Allows any user to approve/deny requests for unowned cats