Change toggle RSVP menu label from "Send email" to "Request reply" (Répondez, s’il vous plaît!)

This sort of fixes #4372 by changing the menu label to indicate how the
underlying code reacts to changes. Of course, it does not resolve that
issue by changing the underlying code to react in the way the original label
promises.

Signed-off-by: Claus-Justus Heine <himself@claus-justus-heine.de>