[lib] Add top-level group chats to GENESIS chat mention candidates object

Summary:
This diff resolves this comment. Chats which have more than two users and are
the first child of GENESIS community are added to GENESIS chat mention candidates object.

Depends on D9218.

Test Plan: Check if first-child chats with type LOCAL are added to GENESIS chat mention candidates object by adding a console.log to chatMentionCandidates selector.

Reviewers: tomek, inka

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D9306