DRY SpecialistDocument show views
DRY SpecialistDocument index views
DRY SpecialistDocument new & edit views
DRY up form errors into a shared partial
DRY attachments forms for specialist documents
DRY up common form partials for js preview

We also DRY up the path helpers for new and persisted previews. This involves a slight change to the path for new previews. As rails by default checks persisted to decide wether to build the path on the collection or as a member, I have pluralised the new paths. This matches closer to rails' standards.
Merge pull request #228 from alphagov/refactor/dry-specialist-document-views

Refactor/dry specialist document views