Ask file provider for URL

Don't reason the URL from implmentation specific magic.

pinaxcon/templatetags/pyconau2017_tags.py

@@ -96,6 +96,5 @@ def trackname(room, day):
 def sponsor_thumbnail(sponsor_logo):
     if sponsor_logo is not None:
         if sponsor_logo.upload:
-            logo_file = os.path.join(settings.MEDIA_URL, str(sponsor_logo.upload))
+            return sponsor_logo.upload.url
-            return logo_file
     return ""