ensure data dir and solr.xml exist at startup

Instead of relying on installation to setup the data directory, set it up at startup. This addresses #165.

This PR moves the template `solr.xml` into `priv/template_solr.xml`, so that it will be packaged with the build. I named it `template_…` because I find that less confusing than seeing multiple `solr.xml` files scattered around.

I've also removed the default index definition from the solr.xml template.