Allow the initiator to close the appointment

[sander]

Kleis: "Initiator of appointment can select a single appointment time for a closed appointment; this automatically notifies all invitees."

Sander: "Here are some quick sketches that would be easily implementable. Currently I think we should combine B and C.

"We will need text for the mail that is sent to participants when the initiator decides on a date."

http://pleft.googlecode.com/issues/attachment?aid=-3808756789275200482&name=pick-date-sketches.jpg&token=LWRUxaP85vu0wBzBjYDJuAFOKO8%3A1324762867301&inline=1

Sander: "English and Dutch text by Kleis at:" https://docs.google.com/document/pub?id=14IL_KIt8l4hyTmnazB4whoEARSoL1n1lXv8z9KUalc8

Work has started on the issue, but then the author unfortunately didn't have time to continue.