Allow customers to upload an image file when creating custom priorities and issue types.

Currently when you create a custom priority in OnDemand you have no option to upload an image file for the icon, you can only insert a URL and that requires you to: store it somewhere external to the OnDemand system (and then have the OnDemand instance dependent on some outside system completely unrelated to OnDemand) or even as a workaround just get an ticket attachment URL and use it there. This could be greatly simplified if a upload icon was added when you create a custom priority.