Design UI for mailbox setup: pairing briar with mailbox app

The Briar app can use one mailbox to receive messages when it doesn't have an internet connection. There needs to be some sort of setup flow integrated into the existing app.

Possibly flow:

1. Feature Onboarding
   • what is a mailbox
   • how can it help me
   • how do I set one up? where to find the mailbox app? link to app
2. Enter single-use setup token shown by mailbox
   • could be a QR code
   • or some briar-mailbox:// type of link
3. (optional) Warn when single-use token was used already and prompt user to generate a new one in mailbox
4. Setup may take up to a minute, show waiting/progress indicator