[FIX] calendar: fix attendees list in invitation email

Upon the creation of an event,
the list of attendees displayed in the invitation emails
were not complete because the emails were sent as the attendees
were being added to the event, in the loop.

e.g.:
 - Create an event with Joseph and Michel
 - Joseph received the invitation email with only him as attendee
 - Michel received the invitaiton email with Joseph and him.

The invitation email of Joseph now contains all the attendees.

opw-681242