Add `src users clean` to src-cli

Created by: DaedalusG

This draft PR tracks work on registering a new src users subcommand src users clean which allows admins to remove users in bulk given some conditions like admin status and days since last active use registered. https://github.com/sourcegraph/sourcegraph/issues/40102

The command is still in development at the time of publishing this draft, it seems near MVP though

Two Additional nice to have features would be:

• output json file containing users removed and some data on the users (for use to send emails to removed users mostly)
• open an editor to compose general email to be sent to removed users and send. Functionality behind -send-email flag

Please check carefully, I'm new at this!

Test plan

Manually test each command option against cse-aws and cse-k8s

Open to additional testing