Send system email when user is add/removed as series admin or stats recipient

As a series admin, I would often ask the eScholarship team to assign new colleagues as admin or stats recipients. However I must tell these new users individually that they have been given (or removed from) the admin role.

An improvement: When a new series admin is added (or removed), they will receive an email notification from the eScholarship system within 1 business day. This way, if they are assigned to multiple series or units, they will receive all notices at once. This saves me time and ensures that no one incorrectly receives a notice if they are accidentally added as admin (e.g. due to typo).