Improve display of alternative download sources

Makes the list of alternative sources displayed all the time
rather than just when the bounding box is too large, and adds
the Overpass API to the list of sources.

Fixes #606
Fixed #635

app/assets/javascripts/index/export.js

@@ -41,6 +41,11 @@ OSM.Export = function(map) {
     $("#minlat").val(bounds.getSouth().toFixed(precision));
     $("#maxlon").val(bounds.getEast().toFixed(precision));
     $("#maxlat").val(bounds.getNorth().toFixed(precision));
+
+    $("#export_overpass").attr("href",
+        "http://overpass-api.de/api/map?bbox=" +
+        $("#minlon").val() + "," + $("#minlat").val() + "," +
+        $("#maxlon").val() + "," + $("#maxlat").val());
   }
 
   function validateControls() {