refactor and reorganize reviewer tools navigation (bug 839400)

https://bugzilla.mozilla.org/show_bug.cgi?id=839400

**Notes:**

- Affects both desktop and mobile.

Screenshots
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![screen shot 2013-07-24 at 2 22 01 pm](https://f.cloud.github.com/assets/674727/851992/71864aec-f4a7-11e2-9031-205c27adf143.png)

![screen shot 2013-07-24 at 2 22 22 pm](https://f.cloud.github.com/assets/674727/851993/7325e240-f4a7-11e2-9aff-a202161a0004.png)

![screen shot 2013-07-24 at 2 24 15 pm](https://f.cloud.github.com/assets/674727/851994/751ae5dc-f4a7-11e2-9f58-a6fb6f497673.png)