Move all splunkjs files into a 'splunk' Django app

Before, we had an appfx root app with all our Splunk-related static files. This
was confusing, as you had paths like appfx/splunk/mvc or appfx/splunk/splunk.js,
and it wasn't clear who owned the styles or contrib packages.

This change moves all those files into a 'splunk' app, which then has a contrib
folder, a css folder, the mvc folder, etc.