Settable and Sortable Navigation Links

Closes #454 

Allow Curated Features, Browse, and About navigation links to be sortable and have their labels settable on the appearances page.

#### Default view
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![label-admin](https://cloud.githubusercontent.com/assets/96776/2620841/79cd4d98-bc45-11e3-8537-e17498a7f916.png)

#### Updated labels
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![updated-labels](https://cloud.githubusercontent.com/assets/96776/2620850/b5b20682-bc45-11e3-8d8e-23d515332b5b.png)

#### About page
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![about page](https://cloud.githubusercontent.com/assets/96776/2620854/c5ba5f52-bc45-11e3-9b56-4cde55aebeb4.png)

#### Browse category list
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![browse-index](https://cloud.githubusercontent.com/assets/96776/2620856/cd936aca-bc45-11e3-9e33-dfd1bc9f1165.png)

#### Browse category view
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![browse-show](https://cloud.githubusercontent.com/assets/96776/2620859/d59f1c3c-bc45-11e3-968d-b0a20b3a1b87.png)