Export public database dumps to BigQuery.

* 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

app/logical/danbooru_maintenance.rb

@@ -15,6 +15,7 @@ module DanbooruMaintenance
     safely { BulkUpdateRequestPruner.warn_old }
     safely { BulkUpdateRequestPruner.reject_expired }
     safely { Ban.prune! }
+    safely { BigqueryExportService.async_export_all! }
     safely { ActiveRecord::Base.connection.execute("vacuum analyze") unless Rails.env.test? }
   end