Flesh out auction complete job

[zachgoldstein]

When an auction is complete, a few things need to happen

• The sale price is calculated (if 10 items for sale, this is the 11th highest bid price)
• A message is sent to everybody that won the auction, includes a link back to the application with transfer instructions
• A message is sent to everybody that bid but lost the auction
• The auction is marked as complete. The bid button is then disabled for buyers

dependent on #5 This should happen out-of-band as a background job, so it can be triggered whenever the auction ends.

This is basically the heart of the application! Very fun