Add border to ani-gifs

- Add border to ani-gifs so they aren't confused with the levels themselves (defined in https://github.com/code-dot-org/dashboard/pull/912 )

![image](https://cloud.githubusercontent.com/assets/206973/3434318/b32b8686-0088-11e4-8705-e210f2727fe3.png)

- Add rounded corners to reshow-ani-gif button

![image](https://cloud.githubusercontent.com/assets/206973/3434321/b69044d8-0088-11e4-9295-f6b1f7b69cd1.png)