Inspect both index and created keys for errored documents

For some reason, when migrating from an unaliased "metasearch",
documents come back with a "created" => key, rather than an "indexed"
key.

I have no idea why. But this stops the rake task from erroring and
allows the aliasing to complete successfully.
Merge pull request #289 from alphagov/allow-inspection-of-created-items

Inspect both index and created keys for errored documents
Add pry
SearchParameterParser is aware of field type
Parsed search parameter hash contains typed values
Parsed params contains filter object
CMA case opened_date should be indexed

In order to filter on this field it needs to be indexed, otherwise
elasticsearch silently returns zero results when trying to do the
filter.
Allow (when specifically requested) schema fields

When a request is made to the unified search with a
`filter_document_type`, e.g.

`/unified_search?filter_document_type=cma_case&fields=case_type,title`

Allow fields to contain schema metadata fields which are to be returned.

With @tommyp
Expand specialist document metadata from schema

When requesting extra fields such as case_type, we want to look up the
label in the schema. This commit does that by passing around the index
mappings, getting the extra fields and looking them up in the relevant
schema.