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
Remove panopticon registration task.

This was only for campaigns, and these should not be registered, as
they're not wanted in search.

This task was not being run on deploy (or at any other time), but its
presence provides confusion, and could lead to mistakes.
Bump slimmer to 3.17.0

To enable us to fix the breadcrumb markup
Remove registerable_campaign as it's no longer needed
Remove G8 summit promo from gov.uk homepage

Reverts 875b006f. See https://www.pivotaltracker.com/s/projects/367813
Downweight gov results to improve top 3 results

The top 3 results are picked by combining all results and picking the three top
scorers. The problem with this is that the scores aren't really comparable. For
many searches, such as "tax disc", the top results are all publications, even
though the most relevant and useful result is from mainstream: "Car tax: get a
tax disc for your vehicle".

This change discounts the government scores by 40%. This will not affect the
ordering of government results relative to each other, just relative to
mainstream/detailed.

Ad-hoc testing of search terms which are currently returning less useful
government results over mainstream results showed that most were improved:
either completely or substantially.

Some results were not improved, such as "esa", "jsa", "maternity", but the
solution for these is probably something else; eg synonyms, content changes,
format weightings.

We were unable to find any cases where we harmed the results where the
government index contains the most relevant results. This did start to happen
if we pushed the multiplication beyond 0.6 - "mod" would stop returning the MOD
organisation page as top result at 0.5.
Scope test to top results

Reduce fragility in case results end up in the non-top-results lists in the
future.
Test that downweighted results are correctly mixed
Merge pull request #369 from alphagov/remove-g8-promo

Remove G8 summit promo from gov.uk homepage
Merge pull request #370 from alphagov/downweight_government_results

Downweight gov results to improve top 3 results
Remove disclosure campaign.

This is service is now live, so this campaign is being redirected to the
real content.
Add placeholder redirect tor unimoney campaign.

A campaign will follow shortly.
Merge pull request #371 from alphagov/unimoney_temporary_redirect

Add placeholder redirect tor unimoney campaign.
Merge pull request #366 from alphagov/remove_panopticon_registration

Remove panopticon registration task.