Displays release date of unit in sidebar

@andy-armstrong @cahrens please review.
I'm a little nervous about the size of the JSON we are going to be returning for the outline page. We render the xblock info for every node in the tree, and they are all going to have the release date from included which is likely to be identical. I'm wondering if it would be better to calculate this on the client side to reduce the size of the JSON returned. One challenge with this idea is that currently the xblock info objects don't have access to their parent info, but that would be simple to add. 

@cahrens Do you have an opinion here? I guess we should just keep it simple and worry about the JSON size in the future.