IDM-05: Link or unlink a non-XSEDE identity

Executive Summary: 
An XSEDE user wants to link or unlink a non-XSEDE identity with their XSEDE identity. A linked identity can be used to authenticate to XSEDE instead of an XSEDE username and password.
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