Tightening up the visual design.

I'm going to be breaking this into a bunch of smaller commits, using this PR to keep running track of this work and updating the description with screenshots as I go.

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### Update design of navigation

![nav](https://cloud.githubusercontent.com/assets/23885/3475220/0238b25a-02ee-11e4-8b46-05ed85d9c2ff.jpg)

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You'd have to leave it here since this file should match ```mkt/commonplace/templates/commonplace/index.html```, but you don't have to render it.