IDM-04: Login to XSEDE user portal with a non-XSEDE identity

Executive Summary: 
An XSEDE user needs to login to the XSEDE user portal (XUP) using a non-XSEDE identity.
User Importance Summary: 
We need a way for the XSEDE system to identify individual human beings. Individual users need to be able to authenticate (prove their identity) to the system, and it is helpful if they can use an existing identity (e.g., their campus login).
Target Communities and Sizes: 
XSEDE users at InCommon or other OAuth2+OIDC institutions - 1000 < N < 9,999