[keyserver] Don't create a thread with an admin when creating a new account

Summary:
On a non-auth keyserver we don't have access to GENESIS and can't create a thread within it.

https://linear.app/comm/issue/ENG-7844/dont-create-a-thread-with-an-admin-when-creating-a-new-account

Depends on D12227

Test Plan: Check if the private thread is created when setting up an authoritative keyserver. Check if the thread isn't created on a non-auth keyserver.

Reviewers: inka, ginsu

Reviewed By: inka

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D12228