[lib] Fix subchannels of subchannels in GENESIS not appearing in navigation side drawer

Summary:
This occurred because I misused the ChatMentionContext. We should use getCommunityThreadIDForGenesisThreads instead of getCommunity, so that channels of GENESIS are treated the same as community roots.

This addresses ENG-8808.

Test Plan: I reproduced the issue locally and confirmed that this diff resolves it

Reviewers: inka, tomek

Reviewed By: inka

Differential Revision: https://phab.comm.dev/D12720