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/post_query_builder.rb

@@ -740,6 +740,9 @@ class PostQueryBuilder
       when :age
         DurationParser.parse(object).ago
 
+      when :interval
+        DurationParser.parse(object)
+
       when :ratio
         object =~ /\A(\d+(?:\.\d+)?):(\d+(?:\.\d+)?)\Z/i
 
@@ -765,6 +768,9 @@ class PostQueryBuilder
         end
 
         (size * conversion_factor).to_i
+
+      else
+        raise NotImplementedError, "unrecognized type #{type} for #{object}"
       end
     end