[keyserver] Delete contained chats when container chat is deleted

Summary: https://linear.app/comm/issue/ENG-4180/deleting-container-chat-doesnt-delete-contained-chats.

Test Plan:

1. Run web app
2. Create subchannel with some threads (sidebars)
3. Check if threads are searchable
4. Delete subchannel and check if threads are still searchable

Reviewers: bartek, tomek, atul, ashoat

Reviewed By: ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8436