Grade level listing page

From Adam on 7/3:

* The grade level page is showing a standard 4-module collection on top of the set of all possible listing items.
* The first module in the list (with the frog next to it) has all of the nested items and can be expanded down to the lesson level. I didn't put the children items in any of the other modules, so those won't expand.
* The all possible listings collection is for markup and style reference, but obviously won't appear on an actual grade level page.
* I added some placeholder 1, 2, 3 icons for the units. I'm planning to refine these down the road, but wanted to include something approximating the style for now.
* The expanding/contracting of the curriculum list elements happens instantaneously for now. This is something that I'd want to revisit so that can happen smoothly. Just an fyi.
* There's a lot of markup on this page to capture the structure of these things and make the nesting work, so if  you want to talk through how things are set up, let's do that. If you want to look at the markup structure for individual curriculum elements, take a look at the possible items list at the bottom of the page. The 4-module collection at the top of the page has things nested, so will be harder to read.