Use Sinatra's url method to generate URLs

By using `url('/')` instead of `set :url...`, we can avoid hard-coding the URL in the source.

I wanted this in dev so I didn't need to change the source if I accessed it through http://127.0.0.1, http://127.0.0.1.xip.io, or [http://localhost](http://localhost), or via a different port.