API v3 articles fixes for Elasticsearch

* Fix searches for tags, categories and date ranges
* ContentBase.search: Add range filter support
* Article: Fix loading attributions back in from ES
Fix category lookup on v2 articles API
Get podcast outputs working with ES

* Index audio attributes 'size' and 'duration'
* Generally work around the fact that `hash.size` == number of keys, not the
  value of the `size` key
* Article: Add `to_es_bulk_operation` convenience method
* Use types for indexing, instead of a `class` attribute in the document