[NEW] postcss parser and cssnext implementation (#6982)

* rocketchat:postcss parser and cssnext implementation

* convert forms, keygrames and reset to cssnext

* convert rtl to cssnext

* convert base.less to cssnext

* convert base.less to cssnext

* fix * selector

* try to fix tests

* add css browser support

* remove duplicated test

* remove duplicated get

* skip fileupload test

* fix tests

* skip fileupload tests

* Try to fix tests

* try to fix the test

* test

* vai que vai

* fix wrong commit

* try again

* try some more

* try to fix tests

* skip the failing tests

* skip last test

* fix stylelint

* stylelint

* fix height calc

* skip sweetalert tests again

* skip more sweetalert tests