Calculate govspeak headers once for CIPs

This means that the headers are only generated once, saving a fair chunk of time
processing the govspeak.
Calculate govspeak headers once for guides
Calculate govspeak headers once for policies
Calculate govspeak headers once for priorities
Calculate govspeak headers once for policy groups
Calculate govspeak headers once for world offices
Calculate govspeak headers once for take part pages
Calculate govspeak headers once for statistical data sets
Calculate govspeak headers once for worldwide organiastions
Simplify use of govspeak_header_contents partial

Instead of passing in the computed headers data-structure, pass in govspeak and
let the partial do the work.
Simplify use of document_contextual partial
Merge pull request #1758 from alphagov/govspeak-headers-calculations

Optimise rendering of govspeak headers on public-facing pages