[lib] introduce unreadCountSelectorForCommunity

Summary:
This diff introduces a new selector that given a the id of a community it will return the unread count for that community. This diff should also address some feedback I got in D10393

Depends on D10391

Test Plan: Confirmed that I was getting the correct unread count for each community in my new nav sidebar

Reviewers: atul, rohan, kamil

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10435