Feature: #87200 - Send plaintext and HTML mails in EmailFinisher

See forge#87200


The EmailFinisher of EXT:form now sends mails with a plaintext and an HTML part. A new option addHtmlPart has been added to configure if a HTML part should be added to mails.


Mails now contain both plaintext and HTML parts to support a wider audience.

Enforcing a plaintext-only mail increases reception and security.