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/controllers/bans_controller.rb

@@ -1,5 +1,5 @@
 class BansController < ApplicationController
-  respond_to :html, :xml, :json
+  respond_to :html, :xml, :json, :js
 
   def new
     @ban = authorize Ban.new(permitted_attributes(Ban))