Bump gds-api-adapters to 10.5.0

This provides access to the rummager unified search API.
Add new unified search controller

This is used only when the "ui=unified" query parameter is supplied to
search.
Enable position tracking in unified search results
Adding a test for the unified search result page

Also removing an unused instance var in the controller action and fixing
an issue with the tests expecting query params to be extracted with a
symbol.

Relates to https://www.pivotaltracker.com/story/show/65973218
Content Api raises an exception with Artefact errors

With the update to gds-api-adapters in this branch, we saw some errors
where Frontend expected to see a 410 if the Artefact had been archived.
This update to artefact_retriever uses the new .artefact! method
introduced with
https://github.com/alphagov/gds-api-adapters/commit/90ee3904 which will
raise an exception for both 404 and 410.
This commit also refactors all the tests that stubbed out .artefact or
had issues with the exceptions being raised.
Remove a double-hidden span

The "No more results for" message had the search term wrapped in a <span
class="visuallyhidden">, but is already entirely contained in a
visuallyhidden h3.  This was copied from index.html.erb, but doesn't
seem to make any sense, so we're removing the inner span tags.
Use clearer variable names

Replace `r` with `result`, and use the `params` hash instead of
assigning request.query_parameters to `qp`, to comply with the style
guide and be more readable.  Use @search_term in search_params hash
instead of params[:q], to reduce duplication of the parameter name
holding the search string.
Extract search #index action to a presenter
Extract search #unified action to a presenter
Merge pull request #551 from alphagov/unified_search_frontend

Unified search frontend

https://www.pivotaltracker.com/story/show/65973218