3 months of events & don't default to sending receipt for past events (CRM-13189)

Many organizations need to register people to past events, but they have enough events that showing all events becomes really cumbersome.  This adds the option to only show events beginning within the past three months when doing a backend participant registration.

Meanwhile, this fixes a small irritating thing where recording a payment for a past event defaults to sending a receipt--most people are aware they already attended!