caching assignment settings for new assignments

fixes CNVS-10388

test plan:
  - create an assignment and set values for the following:
      "grading_type", "submission_type", "submission_doc_types", "points_possible", "assig_group"
      "allowed_extensions", "peer_reviews?", "peer_review_count","automatic_peer_reviews"
      "group_category","grade_group_students_individually", "assignment group", "user notifications",
      "turnitin enabled"
    - save the assignment
    - create a new assignment from the assignment index page with "+ Assignment" button
    - the values you inputted from above should be the default
  - try this again with different values and make sure they save too (esp for binary ones)
  - old assignments shouldn't get these default values (they should keep their old ones)

  - default values should not be shared between courses
  - default values should not be shared between users
    note: default values won't be shared between browsers (no need to test)

Change-Id: Ib5d6200fca795c6dc7ef611570fadc128da083ac
Reviewed-on: https://gerrit.instructure.com/32764
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Hilary Scharton <hilary@instructure.com>