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/concerns/searchable.rb

@@ -212,7 +212,7 @@ module Searchable
       search_text_attribute(name, params)
     when :boolean
       search_boolean_attribute(name, params)
-    when :integer, :float, :datetime
+    when :integer, :float, :datetime, :interval
       search_numeric_attribute(name, params, type: type)
     when :inet
       search_inet_attribute(name, params)
@@ -221,7 +221,7 @@ module Searchable
     when :array
       search_array_attribute(name, subtype, params)
     else
-      raise NotImplementedError, "unhandled attribute type: #{name}"
+      raise NotImplementedError, "unhandled attribute type: #{name} (#{type})"
     end
   end