Support for custom messages and redirects when creating posts (#8434)

* Support for custom messages and redirects when creating posts

When a post/topic is created Discourse serializes a `NewPostResult`
object. Normally this contains a status like `created_post` or
errors describing why the post could not be created.

There are times when a plugin might want to take the inputted post
and do something in the background. In this case, the plugin
can return a custom `message` and `route_to` attribute in the
`NewPostResult`.

If present, the message will be displayed in an alert, and when "Ok" is
clicked the user will be routed to the new URL.

* Destroy the draft in parallel

test/javascripts/helpers/create-pretender.js.es6

@@ -497,6 +497,15 @@ export default function() {
         });
       }
 
+      if (data.raw === "custom message") {
+        return response(200, {
+          success: true,
+          action: "custom",
+          message: "This is a custom response",
+          route_to: "/faq"
+        });
+      }
+
       return response(200, {
         success: true,
         action: "create_post",