[WIP] Initial implementation of ClaimCode.reservedFor.

This doesn't yet alter queries for available claim codes, e.g. the `getUnclaimedCodesTxt` badge route. Nor does it implement email/webhook callback when someone is issued a reserved claim code. It just implements the notion of a reserved claim code in the badge/claim model, along with a new Badge method, `reserveAndEmail()`, which creates a reserved claim code, and changed the `issueMany` badge route to call it.