Dashboard: update from ungraded tracking by group to unpublished tracking

[josh1248]

Related to #2856 and source-academy/backend#965

as noted in #2856 (comment)

requires pairing with backend tweaks as in source-academy/backend#1099

After the changes in the above pull requests, it is now required for assessments to be published before they are visible to students . However, the dashboard still currently reflects only the ungraded submissions out of the total submissions per group. A more useful tracking field for admins would be to track the number of published submissions instead rather than graded submissions - otherwise, this dashboard would not reflect the feedback that the students receive from the avengers. To solve this, the backend route will need to be tweaked in the corresponding backend route, after which the dashboard fields would have to be adjusted accordingly in the frontend.