Revamp the bulk moving of items in the Gradebook setup

Summary:

The current way how bulk moving of items is done in the Gradebook setup is not very intuitive and has some limitations (e.g. does not allow bulk moving of full categories). The aim of this issue is to improve this process and make the experience consistent with the newly introduced bulk move of activities on the course page.

https://www.figma.com/proto/Q32b6Brl54TrEUBTjwgdz0/4.1-Gradebook?page-id=316%3A53496&node-id=6145-158863&viewport=73%2C125%2C0.13&scaling=scale-down&starting-point-node-id=6145%3A159571&show-proto-sidebar=1

Acceptance criteria:

• Bulk move option in the sticky footer area once 1 or more items have been selected
• The bulk move action opens a modal.
• The modal contains a collapsible tree like structure that represents the grade structure (grade categories and items)
• Users can select where the items should be moved.
• The modal has ‘Move’ and ‘Cancel’ actions in the footer.
• Automated testing