Add a new "Add DKDM" dialogue (#1637).

The basic motivation here is to avoid having to tell people to
"just" create a cinema with a screen in it just to be able to make
a DKDM.  Here you can just have a recipient, with emails etc.
and make DKDMs for them.  I hope this makes things clearer from the
user POV even if it does muddy the waters a bit with respect to
DKDMs just being KDMs (really).