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

lib/new_post_result.rb

@@ -9,6 +9,8 @@ class NewPostResult
   attr_accessor :post
   attr_accessor :reviewable
   attr_accessor :pending_count
+  attr_accessor :route_to
+  attr_accessor :message
 
   def initialize(action, success = false)
     @action = action