Dynamic Sublists (Conversations) for GNU Mailman 3

For broadcasting messages, mailing lists are essential but high traffic lists frequently fill the inboxes with replies to threads of a particular topic that subscribers have no interest in. This project introduces Dynamic Sublists into the GNU Mailman 3 suite. This is an ideal feature for user-support community groups or for issue tracking where topics of interests/specific tasks diverge from the main discussion thread. The earlier rigid broadcast model is transformed into an opt-in thread model, ensuring only the root email of a new topic is broadcast to the main mailing list, after which the next round of replies will be routed strictly to a child list, delivering the mail to users who have explicitly opted in. Sublists are treated as dynamically made child MailingList objects linked via a parent_id. This project requires implementation across Mailman 3 Core, extending the SQLAlchemy IMailingList schema, implementing inheritance, building the email command workflow (-new to fork threads and standard -join/-leave aliases), and adding an admin specific toggle for Postorius.