Update 'Requester' to include new_quizzes_url attribute (#684)

[zeeawada]

Proposed Changes

• Add a new_quizzes_url attribute to "Requester"
• Change _get_next_page in paginated_list.py to use an empty string if new_quizzes_url is not defined which resolved the crash.
• Included a test in test_assignment.py called test_get_submissions_without_new_quizzes_url in order to make sure my changes worked.

All tests passed and coverage remained at 100%.

Fixes #684