redirect to dashboard after following masquerade link

test plan:
* as an account admin, visit the user detais page
for a user ("/users/X")
* follow the "Become" link to masquerade as that user
* should be redirected to the dashboard and not
to the previous page (which the current user may
be unauthorized to see)

Change-Id: I2501d86439dece6ef7271832766f13cb4ad23879
Reviewed-on: https://gerrit.instructure.com/45705
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Jahnavi Yetukuri <jyetukuri@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>