Reorder buttons in study options congrats

*Custom Study* and *Unbury* are related and shouldn't be separated by *Open other deck*.

*Open other deck* is probably the most frequently used button, and after a review session, the finger is likely to be at the bottom of the screen. Having the button there as well improves the flow.

In Anki Desktop, *Custom Study* comes before *Unbury*, so I changed that, too:
![congrats](https://cloud.githubusercontent.com/assets/527708/2651459/8f0a0e1c-bf8d-11e3-9bfa-31271cb5593a.png)