mailmerge: generate document bugs

[bobintetley]

The code.mailmerge class has no print, save or PDF targets, but the functionality is there when showing the constructed documents.

jswindowprint needs to be true for mailmerge instead of reading the config (so no need for the print post handler). Save and PDF buttons should be disabled as there's nothing to save to and generating a PDF from potentially a very long HTML document could cause performance problems.

[bobintetley]

html.tinymce_header needs to accept whether save and pdf are enabled as arguments so they can be easily disabled by mailmerge and enabled elsewhere.