Lock Down User Creation To YNet Users, Unless System Admin

[klondikemarlen]

Relates to:

• 0005, Setting User Metadata, 2024-06-06
• Position information populate from directory but need to be overwritten #73

Context

Is your feature request related to a problem? Please describe.
Users should only be able to be created, if they exist in the YNet Active Directory. Or are created by a system admin.

Describe the solution you'd like
Lock down user creation, through sign up, and via user creation endpoint, to YNet Active Directory users, unless system admin.

Additional context
This effects both the "all users" create page /api/users and api/src/middlewares/authorization-middleware.ts -> ensureUserFromAuth0Token`