make document parent IDs term-searchable

Change properties on '*_id' fields to
{'type':'string', 'index':'not_analyzed', 'store':True}
so they can be used for ElasticSearch term queries.
function for deleting whole collections from ElasticSearch

Rather than using ElasticSearch's delete-by-query, this function
loads all the metadata fields under the specified path and
deletes the corresponding documents.
Merge branch '137-browse-ui' of https://github.com/densho/ddr-cmdln into 137-browse-ui