exclude unpublish and ungraded from gb2.5

fixes CNVS-10243

test plan
=========
- turn on draft state, create some unpublished assignments,
  then navigate to screenreader gb
- verify those assignments aren't in the assignment dropdown

- create one or more assignment with a "Not Graded" grading type
- veryify that those assignments don't show up in the assignment
  dropdown

Change-Id: Iaaa52c11bb95c71d19b1a192caadf0a0a45cf162
Reviewed-on: https://gerrit.instructure.com/28210
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Matthew Irish <mirish@instructure.com>
Product-Review: Matthew Irish <mirish@instructure.com>