Add X-UA-Compatible meta tag

Rails 4 removed the `BestStandardsSupport` middleware, because supposedly a HTML5 doctype accomplishes the same thing. But not quite -- without X-UA-Compatible, the UI for changing compatibility mode is still visible, and users can get themselves into a bad state (see https://github.com/openstreetmap/openstreetmap-website/pull/498#issuecomment-28483550)