[server] Script to rename user

Summary: This will change Atul's SquadCal username to atul

Test Plan: I ran the script with a test user in my local environment

Reviewers: atul, palys-swm

Reviewed By: palys-swm

Subscribers: KatPo, zrebcu411, Adrian

Differential Revision: https://phabricator.ashoat.com/D563