[lib/keyserver] convert generatesNotif to return a enum

Summary:
convert generatesNotif to return a enum, either NOTIF or RESCIND, or undefined if the message type should not send a push notif.

Linear Task: ENG-2644

Test Plan: flow also tested chat creation, message creation, message liking/unliking, and adding members to a chat

Reviewers: atul, ashoat, tomek

Reviewed By: ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D6239