get user's display name from IdP

[davepacheco]

Right now, all we have to describe an IdP user is their username. It'd be nice if we supported providing a SAML attribute in the response for their display name. This would presumably involve configuring at the IdP or Silo level which SAML attribute contains the display name, then storing it with user records, and then using it in the console.

(pulled out of #849)