Deserialization of untrusted data in MelisCms

MelisCms provides a full CMS for Melis Platform, including templating system, drag'n'drop of plugins, SEO and many administration tools. Attackers can deserialize arbitrary data on affected versions of `melisplatform/melis-cms`, and ultimately leads to the execution of arbitrary PHP code on the system. Conducting this attack does not require authentication. Users should immediately upgrade to `melisplatform/melis-cms` >= 5.0.1. This issue was addressed by restricting allowed classes when deserializing user-controlled data.