Add K1 large 'next' and 'retry' arrows

Depends on code-dot-org/dashboard#583

For levels marked K1, adds:

![screen shot 2014-05-05 at 4 30 09 pm](https://cloud.githubusercontent.com/assets/206973/2884694/501779aa-d4b0-11e3-898c-0e5c2785a33f.png)

![screen shot 2014-05-05 at 4 30 25 pm](https://cloud.githubusercontent.com/assets/206973/2884698/53a884c4-d4b0-11e3-9e4e-e05f1e7e2c2d.png)