Add support for sending batch emails

[james64]

There is a separate table (let's say EmailStatus) in db with columns regID, lastEmailedStatus. Whenever emailing is triggered it will:

1. Compare EmailStatus with DetailedRegistration
2. Solve all differences by sending emails and updating EmailStatus db

This is run by rest like function which can be called explicitly from code, by java script from web or by cron. We want this to be triggered:

• By new registration POST processing to confirm new registration immediately
• By cron daily to sent info about changes done manually through django-admin
• Optionally put Send emails button to regsys-admin

Maybe this can be used to really send email. Gmail is limiting amount of emails which may result in blockage of outbound emails which we don't want.

This work will be eventually replaced by #4.