[IMP] mail: extended composer for mailing channels

In Discuss, when the user opens a channel, the composer was the same,
regardless of the channel type.  This is fine for chat channels, because
we expect mostly small one liners to be written.  But for mailing
channels, it is a problem:  it is too easy to send a mail to everyone
just by typing ENTER, when the user wanted to go to the next line.

This commit introduces a new widget, the extended composer, to be used
for those channels.  It allows the edition of the subject line, and
don't sent messages when the enter key was pressed.