Use invitation to validate the source of an incoming request

[justinwb]

Handle reverse use case where invitation is used to validate the source of an incoming access request.

• Trevor sends an access request to Bob, masquerading with a webid and profile that makes him look like Alice.
• Bob sees the request, and uses the invitation flow to in reverse to confirm that the request is coming from alice.

cc: @elf-pavlik @angel-araya-jd @ericprud @jamiefiedler