This should be on by default for signatures
filter on display not needed here
[mms] Remove 'compose_cc' and 'compose_bcc' preferences.

Not any compelling reason for these anymore (you are already allowed to
send messages via To, so this doesn't prevent abuse or anything)

If it really needs to be hidden, this can easily be done via CSS.
Allow PDF thumbnails to be disabled
Allow image thumbnails to be disabled
Remove signature hook

The same thing can be accomplished via preference hooks
DIMP -> Dynamic