update attachment#user_id when replacing file content

test plan:
 - have two users in a course that can manage files
   (such as a teacher and a designer)
 - as user A, upload several small text files
 - as user B, on the files page,
   - rename a file
   - move a file to another directory
   - edit the content of a file
 - go to the console and examine the user_id property
   of each of these files.  the one whose content was
   changed should refer to user B; the others should
   remain with user A.

refs CNVS-11219

Change-Id: I8c6b284b3855df52a804f15cd25e514b33be0fb6
Reviewed-on: https://gerrit.instructure.com/33518
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Mark Severson <markse@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>