Don't display HTML mail in the header.

Multi-line html mail causes the whole header to stretch down, boo.
Plus, HTML messages are usually memes from codexbot, status messages,
or long HTML mails from HQ, none of which really benefit from being
in the header. (The long HTML mails will display the status line
in the header, which is not HTML.)

Change-Id: I5892ef65cd407a730117cf4c975b155550d5e17b