Make captcha (kocaptcha) stateless

Also rename seconds_retained to seconds_valid since that's how it is
now. Put it down from 180 to 20 seconds. The answer data is now
stored in an encrypted text transfered to the client and back, so no
ETS is needed

test/captcha_test.exs

@@ -25,16 +25,18 @@ defmodule Pleroma.CaptchaTest do
     end
 
     test "new and validate" do
-      assert Kocaptcha.new() == %{
-               type: :kocaptcha,
-               token: "afa1815e14e29355e6c8f6b143a39fa2",
-               url: "https://captcha.kotobank.ch/captchas/afa1815e14e29355e6c8f6b143a39fa2.png"
-             }
+      new = Kocaptcha.new()
+      assert new[:type] == :kocaptcha
+      assert new[:token] == "afa1815e14e29355e6c8f6b143a39fa2"
+
+      assert new[:url] ==
+               "https://captcha.kotobank.ch/captchas/afa1815e14e29355e6c8f6b143a39fa2.png"
 
       assert Kocaptcha.validate(
-               "afa1815e14e29355e6c8f6b143a39fa2",
-               "7oEy8c"
-             )
+               new[:token],
+               "7oEy8c",
+               new[:answer_data]
+             ) == :ok
     end
   end
 end