Add support for viewing which students have completed a specific task on the Admin dashboard

[mandyWW]

It would be very useful to be able to click on the numbers underneath Total students' task texts and see the names of the students who have completed each task.

[davewhiteland]

As we're finding how useful the texts information is, I think this might be well served richer information beyond the matrix (which is great for progress) — specifically, we do have broadly meaningful created_at and modified_at timestamps on the texts too which might be useful. So I think rather than adding a list of names to the dashboard, I might implement it as a whole page that you can click through which could show richer data than just who but also when.

With a more advanced cohort of students, at some point this ought to be linked into their repos and version control — because the texts ought to be appearing in the Issues text (and perhaps commit messages)... but that's beyond the scope of this little project (something for GitHub classroom instead).

[davewhiteland]

Whilst implementing this I realised it's also probably very useful to see the list of students who haven't authored a text for a specific task, so include that as a toggle

[davewhiteland]

closed: currently implement via the Task text authors button → /admin/task-texts.

It's not great. but it provides a way of seeing who's done what in a different grouping than the alphabetic-order of the text matrix