bans: change expires_at field to duration.

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.

app/logical/bigquery_export_service.rb

@@ -21,7 +21,7 @@ class BigqueryExportService
     Rails.application.eager_load!
 
     models = ApplicationRecord.descendants.sort_by(&:name)
-    models -= [Ban, Favorite, IpAddress, TagRelationship, ArtistVersion, ArtistCommentaryVersion, NoteVersion, PoolVersion, PostVersion, WikiPageVersion]
+    models -= [Favorite, IpAddress, TagRelationship, ArtistVersion, ArtistCommentaryVersion, NoteVersion, PoolVersion, PostVersion, WikiPageVersion]
     models
   end