Tell ElasticSearch which fields to search on at index time
Previously, this was done at search time and at that point we may not know which fields are searchable

For example, when running Page.objects.search(). Pages were only searched on the title field as that is all we know at that time. Specific page types may have defined more searchable fields.

This commit uses the include_in_all property on each field to tell ElasticSearch which fields should be searched on. Then at search time, we don't specify any fields to search on so ElasticSearch will fall back to the ones we defined at index time