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.
* Export daily public database dumps to BigQuery and Google Cloud Storage.
* Only data visible to anonymous users is exported. Some tables have
null or missing fields because of this.
* The bans table is excluded because some bans have an expires_at
timestamp set beyond year 9999, which BigQuery doesn't support.
* The favorites table is excluded because it's too slow to dump (it
doesn't have an id index, which is needed by find_each).
* Version tables are excluded because dumping them every day is
inefficient, streaming insertions should be used instead.
Links:
* https://console.cloud.google.com/bigquery?project=danbooru1
* https://console.cloud.google.com/storage/browser/danbooru_public
* https://storage.googleapis.com/danbooru_public/data/posts.json
