Implement licenses.get_license_by_url

Rather than exploding in the user's face (for example if we custom-configure
licenses in our MG instance, and there are still media with now "unknown"
licenses in the db), simply return a License object as a fallback, where all
attributes are set to the URL we were handed.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>

mediagoblin/db/mixin.py

@@ -139,7 +139,7 @@ class MediaEntryMixin(object):
 
     def get_license_data(self):
         """Return license dict for requested license"""
-        return licenses.SUPPORTED_LICENSES[self.license or ""]
+        return licenses.get_license_by_url(self.license or "")
 
     def exif_display_iter(self):
         from mediagoblin.tools.exif import USEFUL_TAGS