Move links from _metadata partial into new partial

This is in preparation for including the metadata links in the footer.
Initial document_footer_meta on publications
Add change notes to document footer
Fix document footer padding on mobile
Add document meta footer to consultations
Add document meta footer locales
Fix metadata in strong tag not being bold
Style show other content link correctly in footer metadata

This is styled the same as the other links, but might be too prominent. Perhaps it could be normal weight / smaller.
Include document footer on all other document types

Except policies – see https://www.pivotaltracker.com/story/show/49121251
Add document_metadata helper to display document metadata at top and bottom

This was primarily to get an array of metadata into the template so they could be
displayed in the footer as a list instead of a sentence.

It also means the logic of generating metadata has been pulled out into a helper
which can be unit tested.

Some other helpers have been modified to return arrays instead of sentences. Maybe
these don't make sense to be in helpers any longer, I'm not sure.
Add document metadata footer to policies
Fix and simplify document_metadata tests

Tests used to use dom_ids on links to check metadata was working correctly,
but this is brittle and assumes metadata appears once on a page. I have removed
all ids and classes directly on links, and created a consistent set of classes
on the <dd> element which can be used for testing, CSS, etc.