[897] - add an 'ending soon' email for watchers and bidders

[jaspercroome]

Summary

From @bd-viget 's expert guidance, attempting to add an email that goes to watchers and bidders of a given auction when that auction is between 3 and 4 hrs from closing.

• I hard-coded the 3-4 because I was unsure how to access an auction's bid extension window data.
• Ideally, the email would go out once, when an auction is between [bid_extension_window/3] and [bid_extension_window/3] + 1hr
• for now, all our auctions have 12 hour extension windows. so the static values are "accurate". happy to follow-up in another PR with the dynamic values if that's too much of a stretch

Issues

#897

Testing Instructions

Need to deploy to staging to test emails, dev email license is derelict.

[jaspercroome]

@bd-viget I merged this in so that I can move forward with testing. Would still appreciate your eyes on it when you get a chance 🙇‍♂️ 🙏