optimize nullbooleanfields for mobile (bug 917439)

NullBooleanFields don't have ```choices``` attached to them which the mobile search template expects, so I just added from the Django form. The template loops over the ```choices``` of the Django form fields to manually populate a specially-styled dropdown menu. The before is ugly because, in the template, it was set to render as is. I moved it to render with the special styles.

[Here](https://github.com/ngokevin/zamboni/blob/cf795484885ee308d97968a7ed261be52ecf9878/mkt/reviewers/templates/reviewers/includes/queue_search_mobile.html#L53) is where it expects ```choices```

[Here](https://github.com/mozilla/zamboni/pull/1149/files#L0R287) is where I added ```choices``` to the ```NullBooleanFields```

**BEFORE**

![screen shot 2013-09-17 at 11 48 19 am](https://f.cloud.github.com/assets/674727/1159633/c0abf99e-1fc9-11e3-9bc9-cb8d13b9345a.png)


**AFTER**

![screen shot 2013-09-17 at 11 47 48 am](https://f.cloud.github.com/assets/674727/1159635/c1e58e60-1fc9-11e3-9faf-bc3448664962.png)