[CORL-3172]: counts v2 endpoint

[kabeaty]

trafficstars

What does this PR do?

Adds an endpoint for returning comment count for a story.

TODO:

• [ ] More testing of using Redis cache to store counts

These changes will impact:

• [ ] commenters
• [ ] moderators
• [ ] admins
• [x] developers

What changes to the GraphQL/Database Schema does this PR introduce?

Does this PR introduce any new environment variables or feature flags?

yes - COUNTS_V2

If any indexes were added, were they added to INDEXES.md?

TODO Confirm if we need a new index to handle the aggregation count.

How do I test this PR?

You can request the endpoint with something, like this perhaps:

curl -X POST http://localhost:3000/api/story/counts/v2 -H 'Content-Type: application/json' -d '{"storyIDs": ["STORY_ID"]}'

Were any tests migrated to React Testing Library?

How do we deploy this PR?