This makes it mirror how it used to work and how the twig templates
work. The solution is not pretty, but the longer term solution is to
just get rid of the oldui exception.

Also actually run the existing EMail unit tests. That might have
caught this.

Closes: #1269

People might do this in the past to get correct metadata (thereby
breaking other email related functions). But our metadata generator
will now take care of this, so we can just ignore any mailto: prefix.

PSR-12 compliancy

The old mail class uses the built-in `mail` function in PHP, and builds a payload by itself using hard-coded HTML and MIME-snippets. Let's use a library for that; PHPMailer has new dependencies and does all the heavy lifting.