Abillity to collapse rubrics when grading

[LiamClark]

The current implementation has some performance problems on collapsing the table.

fixes #460

[coveralls]

Coverage decreased (-0.2%) to 55.993% when pulling 5725698 on LiamClark:collapse-rubrics into b9badfc on devhub-tud:master.

[Fastjur]

There are indeed some performance issues. Also the state of the arrow seems to be inverted. And the first time we click it it doesn't seem to respond.

Perhaps the initial state is simply incorrect?

[jwgmeligmeyling]

Yup the performance issues are weird. Liam is currently profiling that. It could be that the set of elements is much larger than thought, or that the event handler is invoked multiple times. But it seems the code is just fine and that the problem is actually caused by the fact that collapse is generally slow in Webkit. In the latter case, having this feature not performing is still better than not having this feature at all I suppose...

[TimvdLippe]

I think this query-selector is rather expensive. It might be better to instead add a class collapse-target and do document.querySelector('.collapse-target') and then put the actual value in data-target="${task?index}". You can then retrieve the index with tableRow.dataset.target