Goal: Combine T278190: Introduce Watchlist Expiry Functionality within Discussion Tools and T295090: When I use MassMessage's bot to post a comment, let me auto-subscribe so that I can subscribe to announcements, but not forever.
User story: As a person who sends urgent announcements about upcoming events, I want to send (via MassMessage) and auto-subscribe to hundreds of messages across many wikis. However, when the event is over, I no longer want to have these discussions in my list.