Display a banner to non UK FixMyStreet visitors

only on FMS and only if from outside the uk

Fixes #571
How about something like the following (in base, rather than layout, so mobile gets it too):

```
#country_banner {
    color: $primary_text; background: $primary;
    p#international_message { margin: auto; padding: 0.5em 2em; max-width: 40em; text-align: center; }
    #message_close { float: right; }
}
```

You'd need also a `$('#site-header').css('position', 'relative')` under the `if (data)` in order to have the logos move (currently the banner on mobile appears behind the logos, for example), but this then doesn't obscure the logo/navigation.