Fix handling of strings in SEO and file names

- URLify library updated to latest version (and customizations removed)
- added TextHelper::asciify() method
- TextHelper::sanitizeFileSystem() now does not removes words excluded for SEO
- FileHelper::sanitize() now correctly handles multibyte filenames; if the resulting filename is empty (for instance for Japanese file names) let's build the name from md5()

For a discussion about this: see https://github.com/concrete5/concrete5/pull/1468

- Replaces https://github.com/concrete5/concrete5/pull/1349
- Replaces parts of https://github.com/concrete5/concrete5/pull/1468
- See also https://github.com/concrete5/concrete5-tests/pull/11