Use mailgun open tracking instead of Ahoy

[tdooner]

Open tracking is key to understanding whether we should continue sending to a user. Since it's removed from Ahoy 2, let's reimplement via Mailgun.

• [x] Add new model to replace Ahoy
• [x] Let it run for a while
• [x] Backfill old data
• [ ] Update admin metrics to use new table
• [ ] Remove Ahoy

[tdooner]

Blocks #62