In participantList, we were using the open_! method, which they say
never to use because it can throw NPE.  At the time, didn't know any
other way.  Replaced open_! with for comprehension

In save, make sure the new circle actually got saved before trying to
insert participants.  Not sure this got past me.
In save, we now send an email to the person that was just added to
whatever event.

I forgot to notify people when an event is first created.  The only
people that were getting emails were people that were added AFTER the
circle had been created - that was in RestService.insertParticipants.

I decided to put the email logic inside save so that wherever I needed
to call cp.save, I could be sure that emails would be sent.