Add message on uiser assignment

Not sure why those tests failed.

We should also inject the messenger service.

And the message should probably indicate the nature of the change, such as "added 2 sections" or "removed 1 section" or a list of add/remove.

The form processing doesn't really tell us what was added/removed, so just a confirm is fine.

If you take a look at the README in the -dev branch, you'll see that is not currently required, but we should spin up a new ticket.

tl;dr; Here's what I'm testing with using PHPSTAN (in phpstan.neon):

parameters:
  level: 2
  ignoreErrors:
    # new static() is a best practice in Drupal, so we cannot fix that.
    - "#^Unsafe usage of new static#"
    # Ignore common errors for now.
    - "#Drupal calls should be avoided in classes, use dependency injection instead#"
    # Ignore PHPUnit Prophecy class reflection errors.
    - "#^Call to an undefined method Prophecy#"
  drupal:
    entityMapping:
      access_scheme:
        class: Drupal\workbench_access\Entity\AccessScheme
        storage: Drupal\Core\Entity\ContentEntityStorageBase
      section_association:
          class: Drupal\workbench_access\Entity\SectionAssociation
          storage: Drupal\workbench_access\SectionAssociationStorage

See #3308225: Use dependency injection consistently