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/support/captcha_mock.ex

@@ -6,8 +6,5 @@ defmodule Pleroma.Captcha.Mock do
   def new(), do: %{type: :mock}
 
   @impl Service
-  def validate(_token, _captcha), do: true
-
-  @impl Service
-  def cleanup(), do: :ok
+  def validate(_token, _captcha, _data), do: :ok
 end