change create permission for users

app/policies/user_policy.rb

 class UserPolicy < ApplicationPolicy
   include FollowablePolicy
 
-  def show?
-    record if user_exists?
+  def create?
+    user.is_admin?
   end
 
-  def list?
-    record
+  def show?
+    record if user_exists?
   end
 
 end