Badge sharing

This adds a bunch of functionality for badge sharing, including:

- [x] Middleware for looking up a badge by `public_path` (#334)
- [x] Routes and views for individual badge pages (so far, we've got a route and stub view returning 501 Not Implemented) (#499)
- [x] Stub implementation of "share this badge" button on badge modal that creates a new `public_path` for a badge and provides a link to it.