Issue #3618257: Scope the mail and toolbar alters to what this module owns

The mail alter replaced the first line of every contact page_mail message the site sent, so a site using the core contact form for its own purposes lost the body of its mail. The form a message belongs to cannot tell the two apart - the same form may serve as an HTML page and as an endpoint - so what is recorded instead is who sent it: a small service is open while this module sends, and the alter acts only then. It counts rather than flags, because one submission produces several messages, and it closes again even when sending throws.

The five submission endpoints now send through one method on their base class rather than reaching for the mail handler each on its own.

The toolbar alter returns when the item it decorates is not there, instead of creating a toolbar item that is nothing but an attachment.

Closes #3618257