[keyserver] Introduce make-channel-private.js script

Summary:
For the seed round party, we're going to create an open subchannel of GENESIS that everyone on the app can see.

After the party, we want to hide this channel (make it private). This diff introduces a script that can do that. See step 6 of this plan for more details.

Depends on D4808

Test Plan: I tested this in my local environment and it immediately hid the channel for the test user, both in message history and in the inbox, without crashing the app or causing any issues

Reviewers: atul

Reviewed By: atul

Subscribers: tomek, adrian, abosh

Differential Revision: https://phab.comm.dev/D4809