Add Joomla full site gitignore

Hi,

Here is a new gitignore for Joomla CMS.

Unlike origin `Joomla.gitignore` is use for developing extensions, plugins etc. The new `Joomla-fullsite.gitignore` is for whole Joomla site, If someone want to create a project to track all Joomla files not just an extension, this file will helpful for them.

Thank you.