[keyserver] script to reset a user's password

Summary:
Depends on D13764

script that resets the password for a given user

Test Plan: registered a user with identity, modified staff user list on identity to include my keyserver's user ID, then ran script and changed registered user's password. successfully logged in with new password

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13765