define default article type for ticketcompose

This is a fix for http://bugs.otrs.org/show_bug.cgi?id=10725

Most times, OTRS works correct: When the agent responds to an external email, email-external is selected regardless the translation issue (same for email-internal). An agent can't reply to a note (that is implemented as another note), so the issue is a no-op here. But when someone wants to reply to an outbound-call, you can see the effect described in the bugreport.

The forward dialog already has a default article type.