When accepting/declining a calendar event invitation received by mail,
we landed on an empty gray webclient. The confirmation page had been
dropped with passage to new JS API / new calendar / new webclient.
Previous mechanism had been reimplemented: custom web client with event
information/confirmation if the user is not connected or redirection to
the event form view if the user is connected.
+ FIX manyattendeetag : the status circle did not turn red when an
user declined the invitation.