Initial ExternalSummary model

Really not ready, but should be fairly simple (1-2hrs of focused work).

I'd like to give users the option to add an external summary to ScienceGist. So when they're adding a gist, they would be informed that they can also add a link to a summary that was written elsewhere (blogposts, good news articles, interviews). They would be able to add a link with a title and category (those above), and other users could then vote on these resources just like they vote on gists.

ScienceGist is about understandable summaries of science, I think it's totally cool to link to other resources on the world wide web where this content already exists for a certain paper.

Will have to watch out for spammers, I think anonymous gist submissions will have to be removed when this PR is merged too.