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 57256983210269e8849e61a0866bae4be5033d47 on LiamClark:collapse-rubrics into b9badfc81b2bd5ba905ed71268de6d1907e7705a on devhub-tud:master.

[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...