Changes:
* Change the `expires_at` field to `duration`.
* Make moderators choose from a fixed set of standard ban lengths,
instead of allowing arbitrary ban lengths.
* List `duration` in seconds in the /bans.json API.
* Dump bans to BigQuery.
Note that some old bans have a negative duration. This is because their
expiration date was before their creation date, which is because in 2013
bans were migrated to Danbooru 2 and the original ban creation dates
were lost.
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