change AddReservedUsername to AddReservedUsernames

Summary: We will have a cron job to periodically sync the usernames in Ashoat's keyserver with the reserved usernames list maintained by the identity service. This job will send all the usernames in Ashoat's keyserver to the identity service daily. This change lets us batch taht task, since we can call the RPC just once instead of once per username.

Test Plan: called the updated RPC from keyserver in the child diff

Reviewers: jon, bartek

Reviewed By: jon

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8328