Support adding students to a cohort

TNL-163
Yes, I ran into the same situation that you did, which is that ```view.remove()``` removes the element from the DOM, which is not what you want when the view is constructed with an explicit ```el``` parameter. I was wondering whether we should switch to creating views without specifying an element, because then the view owns its own root. We would then have to explicitly add the view's element into the correct place in the DOM, but then ```view.remove()``` would work correctly.

If we don't do this, we might want to introduce a base view class that handles removing the view like this, because we've already had to do it three times.

For now, I decided to just go with the simple approach.