[ticket/10986] message.id fallback to SERVER_NAME or phpbb.generated

Rather than send invalid message ids with a missing domain part we try
to read one from $_SERVER and otherwise use phpbb.generated

PHPBB3-10986
Merge remote-tracking branch 'naderman/ticket/10986' into develop-olympus

* naderman/ticket/10986:
  [ticket/10986] message.id fallback to SERVER_NAME or phpbb.generated