Merge branch 'html_welcome_mail' into 'stable-3.1'

Allow HTML based welcome messages

I needed a HTML based welcome message, so I added this to mail_plugin.inc.php 

I hope this is the correct approach.

In addition to /conf-custom/mail/welcome_email_DOMAINNAME.txt and /conf-custom/mail/welcome_email_LANGUAGE.txt one may place a /conf-custom/mail/welcome_email_DOMAINNAME.html or /conf-custom/mail/welcome_email_LANGUAGE.html file to allow HTML based welcome messages.

See merge request !276