Share widget

This pull request adds a textarea to the main backpack page that provides a script to embed all the user's public badges into a webpage. Something like 

![this](http://i.imgur.com/40cyg06.png).

This is *not* on a per-group basis – it will embed all badges from all groups that are marked as `public` – which is why it's put on the sidebar rather than into each group.

<!---
@huboard:{"order":3.5}
-->