Add conditional grade column to assignment activity table

[acelaya]

Display a new "Grade" column in the assignment activity table, for assignments where auto-grading has been enabled.

Testing steps

1. Go to https://hypothesis.instructure.com/courses/319/assignments/7296, and log-in as the "Hypothesis 101 Teacher" teacher.
2. Edit the assignment and enable auto-grading (default values are fine).
3. Open the dashboard for that assignment. The table should show a new "Grade" column.

   Note the values of that column may not be displayed yet, as that depends on Calculate and expose auto grading grade on the students metrics endpoint #6664

4. Open the dashboard for any other assignment where auto-grading was not enabled. It should not show the "Grade" column.

[acelaya]

This is starting to get coupled with concrete usages of the component, so we may have to extract it from here.

[marcospri]

Looks good, tested it together with the BE and all seems to line up.

Nothing else to add 👍