Add the API Authentication.

lib/link_shortener_web/auth/guardian.ex

@@ -0,0 +1,16 @@
+defmodule LinkShortenerWeb.Auth.Guardian do
+  use Guardian, otp_app: :link_shortener
+
+  alias LinkShortener.Accounts
+
+  def subject_for_token(user, _claims) do
+    sub = to_string(user.id)
+    {:ok, sub}
+  end
+
+  def resource_from_claims(claims) do
+    id = claims["sub"]
+    resource = Accounts.get_user!(id)
+    {:ok,  resource}
+  end
+end