[lib] Don't refer to children of GENESIS as channels

Summary:
Prefer "chat" since GENESIS children are stand-ins for DMs.

Depends on D7078

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/D7079