Search should not return unpublished entries

There is a passing test for this, `SearchFacetsIntegrationTests#unpublishedEntriesDoNotAppearInResultsOrFacets`, but it was green for the wrong reason (duplicate path).

This commit applies several fixes to how the date `publishAt` is handled in search:

*  `publishAt` is set to `date` in the Elasticsearch mapping.
* A date format is provided to the `Gson` instance in the configured Jest client.
* Filter for unpublished entries are moved to a query filter. When applied as a post filter, the unpublished entries are correctly filtered out from the query result, but show up in the facets.

**NOTE:**  The search index must obviously be rebuilt if this fix is applied to the live system.