[openstreetmap-website] Use `box-sizing: border-box` (#301)

[Saman Bemel-Benrud]

Use `box-sizing: border-box;` instead of default `box-sizing: content-box;` 

Background: http://www.paulirish.com/2012/box-sizing-border-box-ftw/

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Reply to this email directly or view it on GitHub:
https://github.com/openstreetmap/openstreetmap-website/issues/301
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