don't use write_attribute for user_has_posted in streams

refs CNVS-15830

test plan:
 * create a discussion topic that requires a user to post
 * query the stream for that course via the API - the user_has_posted
   field should be true or false, but not nil

Change-Id: I213f26951773301ef31668289eae2fc7f9001d49
Reviewed-on: https://gerrit.instructure.com/41975
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Rob Orton <rob@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>