move cryptographicallySecure into Backend type

This is groundwork for external backends, but also makes sense to keep
this information with the rest of a Backend's implementation.

Also, removed isVerifiable. I noticed that the same information is
encoded by whether a Backend implements verifyKeyContent or not.

Backend/URL.hs

@@ -28,6 +28,7 @@ backend = Backend
 	-- The content of an url can change at any time, so URL keys are
 	-- not stable.
 	, isStableKey = const False
+	, isCryptographicallySecure = const False
 	}
 
 {- Every unique url has a corresponding key. -}