rpc: extend the admin macaroon with signer capabilities

In this commit, we extend the admin macaroon with signer capabilities in
order to allow it to be used with the new signer sub-server. As a
result, users will need to clear out their old macaroons in order to
have the new improved admin macaroon generated. In the future, we may
want to restructure the way the admin macaroon functions: rather than
white listing each of these entities and operations, we can instead add
a catch all capability. This capability will instead allow access to any
call, as each new call would be modified to permit this capabilities and
no others.

rpcserver.go

@@ -129,6 +129,10 @@ var (
 			Entity: "invoices",
 			Action: "write",
 		},
+		{
+			Entity: "signer",
+			Action: "generate",
+		},
 	}
 
 	// invoicePermissions is a slice of all the entities that allows a user