contact_mail() should use renderPlain()

Problem/Motivation

When renderPlain() was introduced, we forgot to update contact_mail() (and maybe other places?) to use renderPlain().

This can be a problem if the contact form is submitted while rendering, in my case, I'm displaying contact forms through an entity reference field.

Proposed resolution

Use renderPlain() instead. Using render() would be wrong, because even if the contact message rendering would add any #attached stuff, we do not want it to end up on the page itself. And we do want to process post render callbacks.

I'm not sure how exactly it should work when you want to send out HTML e-mails, because then you want the #attached stuff to be handled in a separate frame-thingy and put into the HTML mail. But that is a different problem I think.

Remaining tasks

User interface changes

API changes