add user observee api endpoint

test plan:
 - as an admin (with rights) & as the user
 * exercise the observees api
   * /users/:user_id/observees
     * GET (list observees)
     * POST (add observee w/observee credentials)
   * /users/:user_id/observees/:observee_id
     * GET (show observee)
     * PUT (add observee by user id)
     * DELETE (remove observee by user id)
 - the user should automatically gain observer enrollments for courses
   the observees are enrolled in

 - note: only admins with rights can add/remove observees by id

closes CNVS-13400

Change-Id: I5d81bd68212f67281a1e2f3fda54e33d12cbccf1
Reviewed-on: https://gerrit.instructure.com/35868
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>