use css for positioning navigator

[AcrylonitrileButadieneStyrene]

use position: relative; to anchor the unexplored locations icon overlay to the navigator iframe instead of recalculating position in scripting.

tested with this userstyle:

#explorerContainer {
    position: relative;
}

#explorerUndiscoveredLocationsLink {
    left: 0px !important; /* overrides the old scripting behavior, !important not used in commit */
}

closes #412