Create access control handler and create permision

Thanks folks. Would anyone be willing to add a test to the MR?

Someone in another issue posted that http://drupal.org/project/message_ui already defines permissions.

It is odd that a ui module would define permissions.
If you were to disable it would the permissions also go away?

Maybe there should be a change in that module to move permissions into the base message module.

The permissions in the Message UI module are for the Message templates.
https://git.drupalcode.org/project/message_ui/-/blob/8.x-1.x/message_ui....

This will also need a hook_update_N function to add the access handler to existing sites.