[keyserver] Script to delete user

Summary: This script will let us ban spambots like Emilia: ENG-9502.

Test Plan:
I ran this in my local environment, but my user ID isn't 256 on staging, so I got this error:

[Error: user_is_not_staff] { code: 'GenericFailure' }

Once identity 0.41 is deployed to prod, I can try testing this in production.

Reviewers: varun

Reviewed By: varun

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13691