fix 'Become' link for admins on profile page

test-plan:
 - have profiles enabled on the account
 - log in as an admin
 - flag some other target user was "public"
 - try and view that user's profile
 - should not get a 404

Change-Id: Ibcae7f577b5824d42996c830030d5dd87094685f
Reviewed-on: https://gerrit.instructure.com/40250
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Jacob Fugal <jacob@instructure.com>