query the discussion topic materialized view from slave

test plan: load a discussion topic in the web UI. it should load as
before, and if the materialized view already existed, the logs should
show the SELECT for it only against the slave.

Change-Id: If397681f1ebb2502089ee94c24b0292b34ec8eb8
Reviewed-on: https://gerrit.instructure.com/39877
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>