releaseFromQuarantine

Restores the status recorded under DriftQuarantineKeys.PREVIOUS_STATUS and drops both quarantine keys in one save.

Whether a proposition is held is read off its status, so a proposition that isn't PropositionStatus.QUARANTINED has nothing to release and answers null — including one whose quarantine reason was cleared by hand, which is still held.

A proposition with no readable previous status — quarantined by an older policy, or with the key edited away — goes back to PropositionStatus.ACTIVE. Releasing says "let this back into use", and ACTIVE is what that means when the record of where it came from is gone.