shortcuts: Fixed Keyboard shortcut in Firefox.

In Firefox, other Keyboard shortcuts stop working once the users
presses escape to exit the search bar.

Fix this by explicitly focusing the main panel after we exit search.

Fixes #16394.

static/js/message_view_header.js

@@ -150,6 +150,7 @@ exports.exit_search = function () {
         // for "searching narrows", we redirect
         window.location.href = filter.generate_redirect_url();
     }
+    $(".app").trigger("focus");
 };
 
 exports.initialize = function () {