don't re-invite student view student with "completed" enrollment

make the enrollment "active" instead of "invited", or else
User#can_masquerade? will fail because the "invited" enrollment
will not link the student view user to the course via the
courses association

test plan:
 1. have a teacher in an active course who is not an account admin
 2. enter student view
 3. leave student view
 4. conclude the course
 5. enter student view again. it should work this time.

fixes CNVS-13610

Change-Id: If59683a58ae2fcd425a50eccdab8ca74ac435e07
Reviewed-on: https://gerrit.instructure.com/37215
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: James Williams  <jamesw@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>