[lib] Don't mention GENESIS in chat creation notifs

Summary:
Children of GENESIS are meant to be stand-ins for DMs, so when they are created we shouldn't mention GENESIS.

Depends on D7079

Test Plan:
Tested in combination with the next two diffs:

1. Make sure that robotext for chat creation of GENESIS subchannels calls them "chats"
2. Make sure that robotext for chat creation of non-GENESIS subchannels calls them "channels"
3. Make sure that notifs for chat creation of GENESIS subchannels calls them "chats"
4. Make sure that notifs for chat creation of GENESIS subchannels doesn't mention GENESIS

Reviewers: atul, ginsu

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7080