CUMULUS-3639: Add support to db/CollectionSearch to retrieve active collections

[jennyhliu]

Summary: Summary of changes

Addresses CUMULUS-3639: Update /collections/active endpoint to query postgres

Changes

• Updated /collections/active endpoint to query postgres

PR Checklist

• [x] Update CHANGELOG
• [x] Unit tests
• [x] Ad-hoc testing - Deploy changes and test manually
• [ ] Integration tests