[lib] Convert generatesNotifs to function

Summary:
Includes handling the userNotMemberOfSubthreads logic that was previously handled in message-creator.js.

Depends on D6222

Test Plan: Flow, plus tested chat creation, message creation, liking and unliking a user's message, and liking and unliking somebody else's message

Reviewers: ginsu

Reviewed By: ginsu

Subscribers: tomek, atul

Differential Revision: https://phab.comm.dev/D6223