Control character filtering misses leading and trailing whitespace in file and folder names

Summary: It is possible to create files and folders that have leading and trailing `\n`, `\r`, `\t`, and `\v` characters. The server rejects files and folders that have these characters in the middle of their names, so this might be an opportunity for injection.

        556         protected function verifyPosixPath($fileName) {
        557                 $fileName = trim($fileName);
        558                 $this->scanForInvalidCharacters($fileName, "\\/");
        ...
        570         private function scanForInvalidCharacters($fileName, …