Add "MOD Version Check" for extensions to the core

As we already have a composer.json for extensions in place, we can just add the version-file url to it and then compare the currently installed version against the one from the version file and notify the admin about possible updates, when he visits the "Extensions Manager".

Link to the 3.0 MOD: https://www.phpbb.com/customise/db/mod/mod_version_check/