Fixes #6651,bz1094986 - Disable Sync without repo url

Added code not allow syncing on repositories without a URL.
I could see the argument for having a pure authorization method that returns repos that I have permission to sync and a method that attempts to combine the two. However, the latter feels as if it should live in the model, not in the authorization layer.  But I also don't know when you'd ever want to get a list of repositories that you can sync that don't have feed urls.