Fix widget css resources paths.

I played with this a bunch in staging but I couldn't get it to work
using Flask's SERVER_NAME (which seems to be the way it is supposed to
work with url_for).  This way is just as easy, and it works (unlike
url_for).
Move APP_PATH config to the default_config module.
Merge pull request #123 from fedora-infra/feature/fix-widget-css

Fix widget css resources paths.