Use queues for communication with slack

I think this is a great idea.

Some thoughts after a read of the patch:

1. We should document this on the README at least, since its a new feature and has optional integration with advancedqueue by the looks of it.
2. The patch includes a lot of cleanup which is unrelated to the proposed feature, and although they make sense, it makes it a lot harder to review.
3. I think it may be easier if we just added basic core queues integration for now to ease the burden on maintaining this. Integrating with advancedqueue module can be done as an iteration of this if people want it. Up to the maintainers of course.

I believe these to be incorrect since advancedqueue is not a requirement:

1. +++ b/src/Form/SettingsForm.php
   @@ -2,24 +2,51 @@
   +use Drupal\advancedqueue\Entity\Queue;
   

   This will blow up if advancedqueue is not installed.

2. +++ b/src/Slack.php
   @@ -2,42 +2,82 @@
   +use Drupal\advancedqueue\Entity\Queue;
   +use Drupal\advancedqueue\Job;
   

   This will blow up if advancedqueue is not installed.

Add a separate issue about the first point of Proposed resolution in summary:

(1) #3072863: Add the composer file

Changed #2 to point to issue #3170952: Configuration schema is invalid

Patch needs to be re-rolled.

Why add the dependency on the contributed module advancedqueue, instead of using a core QueueWorker plugin?

Needs a reroll

I created a MR from the patch in #13 with some minor re-rolling.

This has been merged. Thank you facine for keeping this dream alive for 8 years. It is a valuable improvement.

This was released as 8.x-1.6