Include Organisation acronym in search index

This lets us remove some ugliness in Rummager that pulls the acronym out from the title.

~~I've left the acronym in the title for fear that removing it will harm the ranking of organisation pages for the organisation acronyms, and then force us to add acronyms for every organisation...~~ The acronym will be indexed and boosted just as title currently is to ensure these pages still rank brilliantly for the acronym.

This has a soft dependency on: https://github.com/alphagov/rummager/pull/125