Dashboard routes: update from ungraded tracking by group to unpublished tracking by group

[josh1248]

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

requires pairing with frontend tweaks as in source-academy/frontend#2898.

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, which is taken from a backend route (see above picture). 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 that returns the dashboard details by group would need to be tweaked to return the number of published instead of graded submissions for use by the frontend, which would need to be adjusted accordingly.

[GabrielCWT]

We should also combine the columns e.g. ungraded missions a/b where a is ungraded missions and b is submitted missions