Only generate slugs for english html attachments.

To prevent slugs being generated with non-ascii characters etc.
Fix html_attachments controller to support ids

friendly_id's find overrides weren't working through the `attachments`
association because friendly_id is only defined on the `HtmlAttachment`
class, not the base class.  Adding the `html_attachments` association
enables this to work correctly.
Merge pull request #1304 from alphagov/fix_utf8_attachment_slugs

Don't generate slugs for non-english html attachments