[Assignment] Move "Mark as done" button to the bottom for assignments

Problem statement:

When assignments have a requirement defined to "Mark as done", the button placement is at the top of the page. This is inconsistent with the location on "Page" items where it was moved to the bottom as noted in Canvas Deploy Notes (2021-07-07).

Proposed solution:

For consistency with other module items such as Pages, please move the "Mark as done" button to the bottom of the page for assignments and have the button adequately distinguishable from the "Submit" button.

User role(s):