FIX: IMAP post alerter race condition and code improvements (#11711)

This PR fixes a race condition with the IMAP notification code. In the `Email::Receiver` we call the `NewPostManager` to create the post and enqueue jobs and sends alerts via `PostAlerter`. However, if the post alerter reaches the `notify_pm_users` and the `group_notifying_via_smtp` method _before_ the incoming email is updated with the post and topic, we unnecessarily send a notification to the person who just posted. The result of this is that the IMAP syncer re-imports the email sent to the user about their own post, which looks like this in the group inbox:

To fix this, we skip the jobs enqueued by `NewPostManager` and only enqueue them with `PostJobsEnqueuer` manually _after_ the incoming email record has been updated with the post and topic.

Other improvements:

* Moved code to calculate email addresses from `IncomingEmail` records into the topic, with a group passed in, for easier testing and debugging. It is not the responsibility of the post alerter to figure this stuff out.
* Add shortcut methods on `IncomingEmail` to split or provide an empty array for to and cc addresses to avoid repetition.

lib/email/receiver.rb

@@ -1184,6 +1184,10 @@ module Email
         options[:post_type] = Post.types[:whisper]
       end
 
+      # To avoid race conditions with the post alerter and Group SMTP
+      # emails, we skip the jobs here and enqueue them only _after_
+      # the incoming email has been updated with the post and topic.
+      options[:skip_jobs] = true
       result = NewPostManager.new(user, options).perform
 
       errors = result.errors.full_messages
@@ -1203,6 +1207,13 @@ module Email
         if result.post.topic&.private_message? && !is_bounce?
           add_other_addresses(result.post, user)
         end
+
+        # Alert the people involved in the topic now that the incoming email
+        # has been linked to the post.
+        PostJobsEnqueuer.new(result.post, result.post.topic, options[:topic_id].blank?,
+                             import_mode: options[:import_mode],
+                             post_alert_options: options[:post_alert_options]
+                            ).enqueue_jobs
       end
 
       result.post