notifications: Send message received from desktop app notification reply.

Adds a electron_bridge event that takes in message id and reply recived from
the notification reply and sends a message. We do this in webapp so desktop
doesn't have to depend on narrow and channel modules.

We also modify zjunit to reset window.electron_bridge after every run
to avoid leaking it.

frontend_tests/zjsunit/namespace.js

@@ -41,6 +41,7 @@ exports.restore = function () {
         delete require.cache[require.resolve(fn)];
     });
     dependencies = [];
+    delete global.window.electron_bridge;
     _.extend(global, old_builtins);
     old_builtins = {};
 };