Wire in latest stories, projects and organizations

On the homepage, wire in latest stories, projects and organizations lists
at the bottom of the homepage template.

I created 3 new template tags to do this: latest_stories,
latest_organizations and latest_projects.

The heavy lifting is done by storybase.utils.latest_context and the
storybase/latest_objects.html template.

I chose 100px for the thumbnail width, because that's the narrowest
thumbnail size offered by Flickr, our most frequent oEmbed provider. To
accomodate this, I added 100px-wide default images as well.

Closes #575, Addresses #433