Contact form only sends e-mails

https://www.phpbb.com/community/memberlist.php?mode=contactadmin currently only sends out an e-mail to one address. This is a rather low effort approach, as only one way of transporting the information is not reliable, with e-mail being the least reliable medium of all.

Sending out the message should use more ways, i.e. XMPP and PM.

Sending out the message should allow more than one recipient. Per medium.

It would be way more reliable when the message goes out as i.e. two separate e-mails (per recipient, which only makes sense when they have different providers, of course), three to five XMPP accounts (if XMPP is used at all) and to one PM (to the predefined administrators group). This way people who complain about not getting any registration e-mail are not caught in a circular trap: when either the user's e-mail provider or the board contact's e-mail provider silently drop e-mails then at least every user in the administrator group knows of the message.

So far I thougth the contact page would act more intelligent, as simple e-mailers have been a problem for decades already once you want to ask for unreceived e-mails and that problem would be known better.

Inspired by https://www.phpbb.com/community/viewtopic.php?f=64&t=2330146&p=14216941#p14216941