pass user info in custom_dashboard url

fixes CAT-70

test-plan:
* in rails console:
* a = Account.root
* a.settings[:dashboard_url] = "http://www.google.com"
* a.save
* login to canvas
* verify that you are redirected to "http://www.google.com?current_user_id=123"
* where current_user_id equals the id of the user you logged in as

Change-Id: I3074f821fa4454b2a568228d00b1da60324b71a7
Reviewed-on: https://gerrit.instructure.com/33225
Reviewed-by: Jon Jensen <jon@instructure.com>
Reviewed-by: Landon Wilkins <lwilkins@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Marc LeGendre <marc@instructure.com>
QA-Review: Marc LeGendre <marc@instructure.com>