Store the digest of the registration key, not the key itslef

Summary: Ref T1536. Like D6080, we don't need to store the registration key itself. This prevents a theoretical attacker who can read the database but not write to it from hijacking registrations.

Test Plan: Registered a new account.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1536

Differential Revision: https://secure.phabricator.com/D6188