Enable notification grouping for notification type "new_response_on_following"

[ayub02]

Description

Extend the existing grouping feature to new_response_on_following notification type so multiple notification for new responses on the same post can be grouped.

Grouping logic will respect SEEN state of the notification, as described here.

Here's the proposed language of the notification:

• Single: {User 1} responded to a post you’re following {post title}.
• Grouped: {User 2} and others responded to a post you’re following {post title}.

Acceptance Criteria

1. More than 1 UNSEEN notifications of type new_response_on_following type for the same post that user is following, should be grouped.
2. All unit tests should pass.
3. All changes are backwards compatible.

[awais-ansari]

We skipped this grouping.

[ayub02]

@awais-ansari we didn't skip it. It's implemented and working.