TODO
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- Make it somehow optional to invite attendees.
  Use a boolean field (checkbox)?

- Currently, the invitation is sent at creation time and in every edition.
  Should it be configurable?

- Should the email message include the event URL?

- If the participant is set with a username, the info message indicating
  that an invitation as been sent should be only show the user id, i.e.
  don't show the email address to any user.


DONE
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- Notify the user that invitations had been sent. Add an info message.
