Events: Make email notifications for NEW events optional

Add a toggle on the Event creation page were the creator can elect to send/not send out an email to users upon creation of the event.
This would be useful because in our community we have multiple recurring, basic events that we don't need to alert the whole community when we add 15 a month. But then we have special events that we want to make people aware of, so then we would elect to send an email when that event is created.