don't break unicode sequenecs when truncating filenames

also, don't round-trip the display_name through the truncated
CGI-escaped filename, since UTF-8 chars become nine-character
sequences, limiting entirely non-Latin filenames to 28 chars
(255 / 9).

fixes CNVS-13426

test plan:
 - all files attached to the ticket should
   (1) upload to the files page without error
   (2) with the filename preserved and untruncated

Change-Id: I41322595e858892313276544607fd3ff2a0310ec
Reviewed-on: https://gerrit.instructure.com/36046
Reviewed-by: Mark Severson <markse@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>