EastSussex. z-index fixes for IE7

The #map_box div wasn't clickable, so we had to reset its z-index from the
broken -1 to 0.  IE7 doesn't like setting to 0 (as it's a falsy value)
so we have to override in CSS as well as position_map.js

See https://github.com/mysociety/FixMyStreet-Commercial/issues/639