UX: Give Admins control over accidental user overages via a `User Cap` admin setting

This is the UX design issue for #328237 (closed)

Note to designer

When this issue is picked up, please advise the BE team on "guidance to provide option to see number of users that will get auto-approved as a result of modifying the User Cap and Require admin approval for new sign-ups settings" per this comment so they can work in parallel. 🙏🏼

Proposal

• Add the number of users being approved to the existing modal
• Only show the modal if 1 or more users will be approved by the settings change