add checkadmin function

lib/diskuy_web/auth/guardian.ex

@@ -45,4 +45,13 @@ defmodule DiskuyWeb.Auth.Guardian do
     end
   end
 
+  def check_admin(user) do
+    case user.role == "admin" do
+      true ->
+        {:ok, :authorized}
+      false ->
+        {:error, :unauthorized}
+    end
+  end
+
 end