Create a post-close anti-tamper hash #92

When a vote is closed, then compute a hash of the vote ciphertext values that were recorded/used for the election (no need to decrypt). Produce this hash during tally.py (or similar) for recording external to the system. (eg. vote-results.json stored elsewhere and seen by voters out of band)

The opened_key protects the election metadata, issues, and mayvote data. The post-close hash protects the vote records from later meddling, as a new hash can be computed and compared to the externally-saved value at any time.