Adds welcome message on registration Welcome message is only send out if podmin account is defined and welcome message is enabled in the config.

This also enables the podmin account to send messages
to any local user.

closes #6128

closes #6118

closes #6117

closes #6111

That is, if it's the first element in the post

closes #6110

This fixes CVE-2015-3226, CVE-2015-3227, CVE-2015-1840 and CVE-2015-3225

regression from #6005

closes #6105