Add formbehavior chosen to selects

This adds the chosen form behavior to the selects in the article modal.

## Current behavior
![skaermbillede 2014-10-05 kl 19 40 36](https://cloud.githubusercontent.com/assets/1738811/4519334/dda7ef6a-4cb6-11e4-96a0-ed80c5d7318b.png)

## After patch
![skaermbillede 2014-10-05 kl 19 40 44](https://cloud.githubusercontent.com/assets/1738811/4519333/dd8dbc30-4cb6-11e4-8655-1be34c4f4477.png)