set Attachment#user_id when uploading files via the API

test plan:
 - upload a file via the API using the POST workflow
 - upload a file via the API using the remote URL workflow
 - examine both Attachment objects via the Rails console.
   the user_id should be set to the uploading user

refs CNVS-11219

Change-Id: Ic61901f7dfac99d74efc57e42e97b90aaf133433
Reviewed-on: https://gerrit.instructure.com/33513
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>