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/policies/ban_policy.rb

@@ -9,11 +9,11 @@ class BanPolicy < ApplicationPolicy
   alias_method :destroy?, :bannable?
 
   def permitted_attributes_for_create
-    [:reason, :duration, :expires_at, :user_id, :user_name]
+    [:reason, :duration, :user_id, :user_name]
   end
 
   def permitted_attributes_for_update
-    [:reason, :duration, :expires_at]
+    [:reason, :duration]
   end
 
   def html_data_attributes