set up Undead Mongoose

[Riscky]

Undead Mongoose will replace the Checkout backend in Koala. This project is WIP, let's track the progress here:

• [x] Build a new Sloth-compatible API
• [x] Write a migration script
• [x] Communicate user data with Koala (using webhooks-light)
• [ ] Build a user friendly admin UI (WIP)
• [x] Authenticate with OAuth
• [x] Write a role to set it up on a server
• [ ] Test the data migration
• [ ] Koala PR to disable the Checkout UI
• [ ] Set Undead up on production
• [ ] Migrate the data
• [ ] Deploy Koala without Checkout UI
• [ ] Configure Sloth to communicate with Undead
• [ ] Make a backup to S3 of all Koala Checkout database data (we'll have to store this for 7 year according to law)
• [ ] Koala PR to remove all remaining Checkout code.

[Phavion]

Oauth is not completely bugfree yet, but it is getting there. Once admin authentication does not create a redirect loop, it is done.