plagiarism_turnitin: Clean uploaded file names before submitting to TII

 * Remove non-ASCII characters from uploaded file names.
 * Truncate uploaded file names to < 100 characters if necessary.

Signed-off-by: Tony Butler <tonyjbutler@gmail.com>
plagiarism_turnitin: Always allow all file types to be accepted by TII

Any restriction of file types will now be taken care of by the Moodle
file upload assignment submission sub-plugin.
plagiarism_turnitin: Reduce max file size for TII submissions to 20 MB