T299657 introduced a generic notification to make people aware when one of the following things happens to a discussion they are subscribed to:

1. The discussion is moved to another page
2. The discussion is merged into another discussion
3. The discussion is deleted

This task involves the work of introducing a new notification type to specifically accommodate the case when a discussion people are subscribed to is merged into another discussion, thereby inhibiting them from receiving future notifications about said discussion.

Story

When a discussion I am subscribed to, and expecting to receive notifications about, is merged into another discussion, I want to be made aware A) that I will no longer receive notifications about said discussion and B) what discussion it was merged into, so that I can decide whether I would like to continue receiving notifications about this topic by way of subscribing to the discussion it was merged into.