[keyserver/web] Enable announcement subchannel creation

Summary: While working on ENG-5182, I noticed that we prevent subchannel creation to be announcement channels. From an initial investigation, it looks like it's been disabled since D5211. An error is thrown when attempting to create a new subchannel with the announcement setting because it fails the input validation.

Test Plan: Ran yarn workspace lib test (to make sure thread-responders.test.js still worked, and more importantly manually tested creating both an open announcement subchannel and a secret announcement subchannel. The permissions like I mentioned are kind of weird though since non-members of the new secret subchannel can discover it in the navigation drawer, so I'll make it part of my overall permissions fixes for ENG-5182.

Reviewers: atul, ginsu, michal, ashoat

Reviewed By: ashoat

Subscribers: ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9489