Allow full-page letterhead

Formerly, the letterhead template only allowed including a banner at the
top of every page.  Now it allows including an arbitrary pdf background.
The background will be placed in the upper-left corner, so top-banner
letterheads should be around 2" x 8.5", and full-page letterheads should
be 11" x 8.5".

(Let me know when this gets merged to mit-current so I can update to our new letterhead.)