Check periodically if a new distribution version is available

Following what was said in XWIKI-14666, the idea here is to check if the environment extension of a given XWiki instance has a new available version.

If a new version is available, we can then fire a an event that will then be handled by xwiki-platform-notifications in order to notify the final user.

Also, we can implement a new xwiki.properties key in order to enable or disable this feature easily.