add feature to filter collection records with in an agency and UI filter to see all child datasets of a parent dataset

Add feature to
- [ ] filter collection records with in an agency
- [ ] represent the parent records through an icon on the organization page
- [ ] UI filter to see all child datasets of a parent dataset

Currently it is possible to identify collection records and their child records through custom search queries:

Query for finding all parent datasets : https://inventory.data.gov/organization/small-business-administration?q=is_parent%3Atrue&sort=score+desc%2C+metadata_modified+desc

Finding the CKAN ID of a given parent dataset: https://inventory.data.gov/api/rest/dataset/small-business-profiles-for-the-states-and-territories-2014-20160204113221-2016041911-20161102103122
"id": "76b2f886-beb9-4d93-a7ca-cdc85cdf64ea"

Finding the child datasets which have the above ID as the parent dataset in their metadata: https://inventory.data.gov/organization/small-business-administration?q=parent_dataset%3A%2276b2f886-beb9-4d93-a7ca-cdc85cdf64ea%22&sort=score+desc%2C+metadata_modified+desc

The issue provides no repository files or tests; start with the organization search examples and the dataset REST API URL in the report. Trace how parent and child dataset metadata is searched and rendered on the organization page. Done means agency filtering, parent-record icons, and a UI filter for all child datasets work together.

Written by the indexing model from the issue text.