spec: refactor graded discussion/quiz spec

rewrote helper methods for building and editing assignments in draft state.
rewrote all tests in assignments_discussion_spec and asssignments_discussion_spec
to use new helper method.

Change-Id: If2b619723cfa7c04510d4e233f912ecea482dae1
Reviewed-on: https://gerrit.instructure.com/42931
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cameron Sutter <csutter@instructure.com>
Product-Review: Kyle Rosenbaum <kyler@instructure.com>
QA-Review: Kyle Rosenbaum <kyler@instructure.com>