Using the rummager elastic search API for searching

Depends on these pull requests being merged:
https://github.com/alphagov/rummager/pull/53
https://github.com/alphagov/gds-api-adapters/pull/44

We also need to add smokey tests to ensure indexing and searching works in preview.

We would need to destroy and recreate the government index as there has been a change from id's to slugs in existing keys, then reindex.