Improve Govspeak presenting with cached sanitising
Improve Govspeak presenting with cached sanitising

This changes how we lookup atttachments from a big loop to instead a
cached hash. This improves performance on documents with large numbers
of attachments.

alphagov/specialist-publisher-rebuild,alphagov/specialist-publisher-rebuild,alphagov/specialist-publisher,alphagov/specialist-publisher,alphagov/specialist-publisher,alphagov/specialist-publisher-rebuild,alphagov/specialist-publisher-rebuild