[identity] implement privileged delete users RPC

Summary:
check if caller is staff. if they are, then delete the users whose IDs are provided in the request

Depends on D13629

Test Plan: tested in next diff by calling RPC

Reviewers: bartek, will

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D13630