Separate toasts for existing & new device verification

Separate device verification toasts into ones for devices that were
there when the app loaded and a separate toast for each device that
has appeared since.

Reverts part of https://github.com/matrix-org/matrix-react-sdk/pull/4506
(clicking a device from your own UserInfo now triggers the legacy
verification flow again).

Fixes https://github.com/vector-im/riot-web/issues/13422
Fixes https://github.com/vector-im/riot-web/issues/13418

test/utils/ShieldUtils-test.js

@@ -11,7 +11,7 @@ function mkClient(selfTrust) {
         checkDeviceTrust: (userId, deviceId) => ({
             isVerified: () => userId === "@self:localhost" ? selfTrust : userId[2] == "T",
         }),
-        getStoredDevicesForUser: async (userId) => ["DEVICE"],
+        getStoredDevicesForUser: (userId) => ["DEVICE"],
     };
 }