Add SignedMessageGuard.
Add SignedMessageGuard.

SignedMessageGuard verifies that the message has a signer, and blocks
any unsigned messages.

tcmitchell/geni-portal,tcmitchell/geni-portal,charliemeyer/geni-portal,charliemeyer/geni-portal,tcmitchell/geni-portal,tcmitchell/geni-portal,charliemeyer/geni-portal,tcmitchell/geni-portal,charliemeyer/geni-portal,charliemeyer/geni-portal