[keyserver] Create messages for open GENESIS subchannels

Summary:
To avoid spammming everybody, today we don't create messages when a subchannel of GENESIS is created.

But for the seed round party, we want to create an open subchannel of GENESIS that everybody can access. To make it easy to access, we want to have a message appear in GENESIS about the channel.

This diff makes it so the messages are still created for open subchannels of GENESIS. The only way for an open subchannel of GENESIS to be created is for me (@ashoat) to do it manually, so this is safe.

Test Plan: I tested this in my local environment and it created a message (but only when combined with the hacks listed here)

Reviewers: atul

Reviewed By: atul

Subscribers: tomek, adrian, abosh

Differential Revision: https://phab.comm.dev/D4808