Redesign grader report single view bulk change UI

The current UI for bulk changes is confusing and clunky. It needs a complete re-think.

• It is not clear if it works on all users or just the ones on the current page
• The same save button is used for both bulk changes and individual changes
• If I make individual changes and check the bulk changes checkbox my changes are lost
• It is not visually separated from the rest of the page