Fix the change description boxes to allow collapsing/expanding.

Review boxes were made collapsable/expandable, but this didn't end up
applying to change description boxes.

Now there's a base view called CollapsableBoxView that handles
everything having to do with collapse state. ReviewBox inherits from
this.

ReviewBoxListView sets up a CollapsableBoxView for every change box on
the page.

Tested expanding/collapsing individual review boxes, change description
boxes, and all at once.