keep pdf,png,bubble-json in database

[brianolson]

Keep rendered pdf,png and bubble.json in database. In case of rendering changes in draw.py, this would allow a scan to be compared against the archived png and bubble.json.
Stretch goal: keep multiple versions?
Stretch goal: mark a specific version as 'official', something that should really really be protected because it's being used in an actual election. Archive the source json along with pdf,png,bubbles.