Added landing page.

* Separated footer and sock/feedback.
* Root url will redirect to /landing.

This is an early PR that adds a landing page to Insights.  The cover image is TBD, and this PR will be updated when that is available.

Example with interim cover image:
![landing-page](https://cloud.githubusercontent.com/assets/5265058/5168348/5518d24e-73cc-11e4-8b20-b06df63b6773.png)


@rocha @clintonb 
Updated to `audience_message_column_width `.