[lib] Handle [leave/delete]ThreadActionTypes.success in reduceThreadActivity

Summary: We want to remove entries from ThreadActivityStore once the threads have been removed.

Test Plan: Navigate to thread, log ThreadActivityStore, delete thread, log ThreadActivityStore.

Reviewers: ashoat, ginsu, tomek

Reviewed By: ashoat

Subscribers: wyilio

Differential Revision: https://phab.comm.dev/D9402