Add Aside

PLAT-216

@cpennington here's the sketch. I'll try to figure out how to use workbench tomorrow. Feel free to comment, educate, ...
ah, I didn't impl passing the xblock dom to the aside js object init fn. I guess that's a rqmt? It does pass the xblock's usage id to the aside's `render_template` and adds it to `$(aside).data('block_id')` (and the aside as `self` in `render_template` and as `$(aside).data('usage')`).

The xblock's usage id is passed to the aside's server side views and handlers but the xblock itself isn't.

I can add tests for the above as impl'd