don't use write_attribute as temp storage for conversation stream items

refs CNVS-15830

test plan:
 * involve yourself in a conversation
 * on your dashboard, the last message and last author info should be
   correct and not asplode

Change-Id: I997be1a0f40939d1999ffbc99ec7b28ed322ffbc
Reviewed-on: https://gerrit.instructure.com/41976
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>