Allow cliend-side caching

Add Expires and Cache-Control headers to API responses. This allows
browsers to cache them and avoid extra round-trips for data. Also use
plain JSON type in UI to allow browser to use cache. For widget to work
added "Access-Control-Allow-Origin: *" header to all API requests. Note
that althogh this will make cross-origin requests work, it's still not
enough to make browser cache such response, so widget won't benefit from
client-side caching yet.

Change-Id: I49eeffccaf79fe3417bd941590b5dd239ddc433f

stackalytics/dashboard/static/js/stackalytics-ui.js

@@ -89,7 +89,7 @@ function renderTableAndChart(url, container_id, table_id, chart_id, link_param,
 
         $.ajax({
             url: makeURI(url),
-            dataType: "jsonp",
+            dataType: "json",
             success: function (data) {
 
                 var tableData = [];
@@ -325,7 +325,7 @@ function initSingleSelector(name, api_url, select2_extra_options, change_handler
 
     $.ajax({
         url: api_url,
-        dataType: "jsonp",
+        dataType: "json",
         success: function (data) {
             var initial_value = getUrlVars()[name];
             if (initial_value) {