Add specialist sector info to document headers.

Includes:
- Primary sector in document type header
- Primary subsector in document title header
- All (sub)sectors in metadata
Add sector metadata translation keys.

Separate commit to reduce PR noise.
Move generic document step to document_steps.rb
Merge pull request #1373 from alphagov/add-specialist-sectors-to-document-headers

Add specialist sectors to document headers

https://www.pivotaltracker.com/story/show/66346196