associate with shard when masquerading (for db page views)

test plan:
 * enable db page views
 * as an admin in shard 1, masquerade as a user in shard 2
 * view the user in shard 2; the page views from the admin should
   show up

Change-Id: I3c71ab12d207159d2f2dd0985449f4576bb04b67
Reviewed-on: https://gerrit.instructure.com/28355
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Reviewed-by: Nick Cloward <ncloward@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>