Support second domain for outgoing emails

this splits DEFAULT_EMAIL_HOST into NOREPLY_EMAIL_HOST which will be used as the sender for mass-mail style emails that should not get a response (e.g. noticiations and mass mailer) and PLATFORM_MAILBOX_HOST for user/group facing mailboxes.

to allow the setup of using noreply@foodsharing.de as a sender address for the big bunch of mails we are sending ans xyz@foodsharing.network for all platform mailbox accounts.