add login button
'rails g controller Sessions new'
'rails g mongoid:config'
'rails g scaffold User name:string password:digest'

We've removed the following *.jbuilder files as we won't be needing
them:

        app/views/users/index.json.jbuilder
        app/views/users/show.json.jbuilder
move navbar to its own file '_navbar.slim'
add login link/route

The line in routes.rb is necessary because it creates the 'login_path'
variable for us.
add 'login' page (sessions#new)
use bcrypt and configure User model for mongodb

In particular, we cannot use the "Digest" type because mongodb does not
have that as a native type; instead, we use the String type, and rename
the field to "password_digest".
seed: create root user from environment variables
add user login
add login/logout functionality