Backend: creating a user management admin

[arath7]

Right now, we have a super admin that can do task template and study builder editing as well as user management. We want 3 admins total: a super admin that does both, an admin for just study builder component editing, and an admin for just user management. This ticket will be the backend for creating the user management admin.

• Add a role field to the userSchema to distinguish between different admin types (for this ticket, just distinguish between super admin and user management admin) in models/user.ts
• Modify the isAdmin middleware to enforce permissions based on roles in middleware/auth.ts (like a hasRole)
• Update the admin.route.ts for role based permissions. Modify/update existing routes to restrict access based on roles. Super admins can do everything but user admin can only use routes related to access control (adding/removing users)