DA - more refactoring

- cleans up assignment model
- eliminates unnecessary feature flag checks
- eliminates unnecessary teacher permission checks
- moves duplicated methods from assignment and quiz
  to new module

Change-Id: I8d1e8f99bf5c8f6a63bd670b432aeef7e8f0e86b
Reviewed-on: https://gerrit.instructure.com/41654
Reviewed-by: Cameron Sutter <csutter@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Sean Lewis <slewis@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>