ECMS-4895: Adding @import in Site CSS crash the server

Problem analysis
	- In case of file whose extension is css e.g example.css in css folder for a site, server will create example.css containing all active css file including example.css. If there is another file imports example.css, server will create many files example.css then integrate to css file for portal. This progress is unfinisable.

Fix description
	- Create a filter which accepts only Stylesheet.css in Default folder of a specific site