[#3] Add selectbox to pick the GeoJSON resource

Even though we show the resource name, we're saving just its URL. This means
that if the user re-uploads the resource, this will point to the old URL and
will have to be updated as well.

I haven't saved the resource id itself for performance reasons. If we didn't
had the URL, we'd have to get it before loading the map. Every time. We might
want to do this way in the future, anyway, as the performance might be
acceptable when it's cached.
Load viewhelpers when running tests

We depend on viewhelpers/main on our resources.config. It would be great if we
were able to mock it out in the future, as we're not testing JS anyway.
[#14] Style the remove field "X" icon