return file's uploading user in api

test plan:
 - consult api docs for files index and show actions
 - upload some files via the files page, putting files
   up with at least two different users
 - do index with ?include[]=user
  * user who uploaded each file should be returned
 - do show with ?include[]=user
  * user who uploaded the file should be returned

fixes CNVS-11534

Change-Id: I973f96e67d80a5c000482c60d57f928004901f2a
Reviewed-on: https://gerrit.instructure.com/33622
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Mark Severson <markse@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Reviewed-by: Ryan Shaw <ryan@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>