DA - observer refactor

refs CNVS-15158

test plan:
  - general regression test as observers for selrel
  - things to look at: assignments, quizzes,
    discussions, & module index pages, quizzes api

Change-Id: I1211c58ebbd8bc0cd32610fafad215d11af58e16
Reviewed-on: https://gerrit.instructure.com/41039
Reviewed-by: Simon Williams <simon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Sean Lewis <slewis@instructure.com>
Product-Review: Mike Nomitch <mnomitch@instructure.com>