Add a link to a new artist from the dashboard.

I like writing tests like these, even though they seem kinda trivial, because
if I do visual refactoring of the HTML/CSS/JS later, I can ensure that I don't
break anything.

I've also moved the `sign_in` helper into a more global scope, as an increasing
number of tests will use it.

Finally, you'll notice that I use `only` in my routes more than most people.
It's true that those will just be a 404 if they're not defined, but I like
being extra explicit here. For example, `rake routes` would show waaaay more
routes than I actually use. Routes are an interface into your application: you
should limit them as much as possible.