don't display account settings notification tab if empty

fixes CNVS-2684
fixes CNVS-2768

test plan:
- in the site admin account settings page:
  - the notification tab should be gone
  - updating settings should still work
- in the root account settings page:
  - the notifications tab should still be present and functional
- in a sub-account settings page:
  - the notification tab should be gone
  - updating settings should still work

Change-Id: Ic6b016be8d0b7a00f4153870220dbb34388a350b
Reviewed-on: https://gerrit.instructure.com/38367
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>