Add condition for signed-out state to link to sign in / get API key

[louh]

If a user is not signed in, or does not have any API keys, the your-mapzen-api-key should be replaced with an interactive element that prompts a user to sign in or obtain an API key. A popover might be used to accomplish this, but would need to be enabled in the styleguide. See https://github.com/mapzen/styleguide/issues/496