Cache User#is_admin? & User#is_staff?

is_admin? is called in many places, often 3 and more times
per action.

Co-authored-by: Ana María Martínez Gómez <ammartinez@suse.de>

src/api/app/models/user.rb

@@ -385,11 +385,13 @@ def update_user_info_from_proxy_env(env)
   #####################
 
   def is_admin?
-    roles.where(title: 'Admin').exists?
+    return @is_admin unless @is_admin.nil?
+    @is_admin = roles.where(title: 'Admin').exists?
   end
 
   def is_staff?
-    roles.where(title: 'Staff').exists?
+    return @is_staff unless @is_staff.nil?
+    @is_staff = roles.where(title: 'Staff').exists?
   end
 
   def is_nobody?