Add support for specialist document attachments

Specialist documents can have multiple attachments with titles. We capture the original filename so that we can present markdown hints for embedding the attachment in the admin UI.
Tidy up safe_html_validator_test

it's cleaner to use `#select` than mapping to possibly nil values and then compacting.

Also allow SafeHtml validator to be used on model classes which don't define GOVSPEAK_FIELDS, as it seems superfluous to define this on a model which doesn't have any fields.
Validate Attachments with SafeHtml validator
Attachments must expose URL
Ensure attachment callbacks fire on edition save
Stub asset_api_client rather than using webmock

Also tightened up the tests on specialist_document_edition to validate
that the file upload does get triggered when saving the
SpecialistDocumentEdition (verifying that the callbacks get fired on the
embedded attachment document).
Merge pull request #133 from alphagov/add-attachments-to-specialist-document-edition

Add support for specialist document attachments