Migrate wiki articles to Vaadin documentation (#9912)

* Vaadin Tutorial For Swing Developers
* Setting And Reading Session Attributes
* Enabling Server Push
* Cleaning Up Resources In A UI
* Sending Email From Java Applications
* Using Parameters With Views
* Optimizing Sluggish UI
* Configuring Push For Your Enviroment
* Setting And Reading Cookies
* Using Polling
* Creating An Application That Preserves State On Refresh
* Finding The Current UI And Page And Vaadin Session
* Sending Events From The Client To The Server Using RPC
* Handling Logout
* Remember To Set The Locale
* Scalable Web Applications
* MVC Basics In ITMill Toolkit
* Access Control For Views
* Customizing The Startup Page In An Application

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+[[finding-the-current-ui-and-page-and-vaadin-session]]
+Finding the current UI and page and vaadin session
+--------------------------------------------------
+There are many cases where you need a reference to the active `UI`, `Page`
+or `VaadinServiceSession`, for instance for showing notifications in a
+click listener. It is possible to get a reference to the component from
+the event and then a reference from the component to the UI but Vaadin
+also offers an easier way through two static methods:
+
+[source,java]
+....
+UI.getCurrent()
+Page.getCurrent()
+VaadinSession.getCurrent()
+....
+
+For example when you want to show the name of the current UI class:
+
+[source,java]
+....
+Button helloButton = new Button("Say Hello");
+    helloButton.addClickListener(new ClickListener() {
+        @Override
+        public void buttonClick(ClickEvent event) {
+            Notification.show("This UI is "
+                    + UI.getCurrent().getClass().getSimpleName());
+        }
+    });
+....
+
+Similarly for `VaadinServiceSession`, for instance to find out if the
+application is running in production mode:
+
+[source,java]
+....
+public void buttonClick(ClickEvent event) {
+    String msg = "Running in ";
+    msg += VaadinSession.getCurrent().getConfiguration()
+            .isProductionMode() ? "production" : "debug";
+    Notification.show(msg);
+}
+....
+
+And finally similiarly for `Page`. For instance adding a browser window
+resize listener can be added like this:
+
+[source,java]
+....
+javaPage.getCurrent().addBrowserWindowResizeListener(
+  new Page.BrowserWindowResizeListener() {
+    @Override
+    public void browserWindowResized(BrowserWindowResizeEvent event) {
+      Notification.show("Browser resized to " + event.getWidth() + "x" + event.getHeight());
+    }
+});
+....
+
+*Note* that these are based on `ThreadLocal` so they won't work in a
+background thread (or otherwise outside the standard request scope).