Changed .drop_choices z-index from 100 to 101

I am fixing [issue #1175](https://github.com/reddit/reddit/issues/1175#issuecomment-63179480). 

The subreddit dropdown menu would appear behind the subreddit selector text area (on the submit page), and it is because they both have a z-index of 100. So I changed the z-index for the dropdown menu to 101.