[keyserver/lib] Populate special_role field when initial roles are created

Summary:
Now that we've createde a roles.special_role column in MariaDB and wrote a migration that populates it for existing threads, we should make sure new threads going forward populate this upon creation.

Depends on D9961

Part of ENG-5833

Test Plan: Create some new communities and make sure the special_role is populated for Members (no special role for Admins yet)

Reviewers: atul, ginsu, tomek

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D9974