Link to the first mainstream spelling suggestion

This is (probably) firing a duplicate search query, I'll refactor that shortly.

CSS, markup and accessibility advice courtesy of @markhurrell and
@workwithgreat.
Hide spelling suggestion with feature flag

The spelling suggestion currently generates rude words, so we need to consider
how to deal with that before we can make this public.
Avoid duplicate mainstream request

Caching may have prevented an actual duplicate request, but you'd have to know
detail about gds-api-adapters to even guess that. Anyway, the cache is
generally disabled in development.

Additionally make it backwards compatible with Rummager before spelling
suggestions were added.
Retain spelling suggestion feature flag on search
Retain tab and spelling feature flag on suggestion
Fix brittle test broken by last commit
Merge branch 'master' into spelling_suggestions

Conflicts:
	app/controllers/search_controller.rb
Tidy up tests
Merge remote branch 'origin/spelling_suggestions'