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/acceptance/composer-test.js.es6

@@ -234,6 +234,26 @@ QUnit.test("Create an enqueued Topic", async assert => {
   assert.ok(invisible(".d-modal"), "the modal can be dismissed");
 });
 
+QUnit.test("Can display a message and route to a URL", async assert => {
+  await visit("/");
+  await click("#create-topic");
+  await fillIn("#reply-title", "This title doesn't matter");
+  await fillIn(".d-editor-input", "custom message");
+  await click("#reply-control button.create");
+  assert.equal(
+    find(".bootbox .modal-body").text(),
+    "This is a custom response"
+  );
+  assert.equal(currentURL(), "/", "it doesn't change routes");
+
+  await click(".bootbox .btn-primary");
+  assert.equal(
+    currentURL(),
+    "/faq",
+    "can navigate to a `route_to` destination"
+  );
+});
+
 QUnit.test("Create a Reply", async assert => {
   await visit("/t/internationalization-localization/280");
 

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",