signing: Store bare signature from autograph, not content-signature

[mythmon]

This changes the signatures stored in the database from x5u=URL;ecdsa384=SIG to just SIG. The rest of the data is already stored in the DB.

This will require changes to the system addon, since it was built against the longer version.

[Osmose]

Code changes look good, but the Autographer tests do not test the handling of the response coming back from Autograph, which is why this change doesn't cause test failures. We should be testing that.