Make the checkbox "Show description" checked by default.

#6110 

When creating a new term in a vocab named "foo" the checkbox is checked by default:
![selection_403](https://cloud.githubusercontent.com/assets/4497748/3671090/caff9cf0-124b-11e4-8909-a21672181f62.png)

And the term description is displayed on its page:
![selection_404](https://cloud.githubusercontent.com/assets/4497748/3671098/d88e9204-124b-11e4-8583-327b22738d07.png)