add observees page to user profile

test plan:
 * navigate to /profile/observees
 * add new observees (specifying usernames & passwords)
 * new observees should appear in the list

notes:
 - the 'Observing' tab does not appear until at least one observee has
   been added for the user (and the page is refreshed)
 - the observing user should gain Observer enrollments for courses the
   observed user is enrolled in (both present and future)

closes CNVS-13356

Change-Id: Icda0f0d256fc73bcb554523db6436457941d8b58
Reviewed-on: https://gerrit.instructure.com/37283
Reviewed-by: Ryan Shaw <ryan@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Hilary Scharton <hilary@instructure.com>