fix files regression in IE8. fixes #11109
fix files regression in IE8. fixes #11109

files tree wasn't loading properly in IE8 because full_files.js was
using [].indexOf, which isn't supported in IE until version 9.

test plan:
  * using IE8, open course or personal files page;
  * verify that folders and files appear as expected and that the
    page properly functions.

Change-Id: I60f4a62205d36cf08786004c5dcdfeda55f9f5b9
Reviewed-on: https://gerrit.instructure.com/14332
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Sterling Cobb <7a57d889ea7f266299434459898a90c7f6a1ca4d@instructure.com>

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