Add Compass gem and style signup form.

Modelled after http://getbootstrap.com/examples/signin/.
Server-side login session controller.

Modelled after https://www.railstutorial.org/book/log_in_log_out.
Login form side-by-side with signup form.
Require authentication before adding a page to an album.
Login form AJAX request. Include CSRF token.
Stop making requests for empty photo URLs.