Don't send mail to user who initiated transition

Problem/Motivation

The "Last revision author" recipient type sends a notification to the author of the previous revision. This works great in some situations, for example when a reviewer sends content back to the author. It can be a bit noisy when the author is transitioning their own content, as they are often the author of the previous revision.

Steps to reproduce

Proposed resolution

Update \Drupal\workbench_email\Plugin\RecipientType\LastRevisionAuthor::prepareRecipients to filter out the current user's email address. Probably should be configurable too.

Remaining tasks

Decide if this makes sense for other recipient types too.

User interface changes

API changes

Data model changes