Made sitemap.xls caching configurable

refs https://github.com/TryGhost/Toolbox/issues/411

- Having hardcoded cache control values in the codebase makes it impossible to experiment with new values without a version release.
- Having all values configurable by default will allow for easier caching experiments and customizations on self-hosting instances.

ghost/core/core/frontend/web/site.js

@@ -65,7 +65,7 @@ module.exports = function setupSiteApp(routerConfig) {
     siteApp.use(mw.serveFavicon());
 
     // Serve sitemap.xsl file
-    siteApp.use(mw.servePublicFile('static', 'sitemap.xsl', 'text/xsl', constants.ONE_DAY_S));
+    siteApp.use(mw.servePublicFile('static', 'sitemap.xsl', 'text/xsl', config.get('caching:sitemapXSL:maxAge')));
 
     // Serve stylesheets for default templates
     siteApp.use(mw.servePublicFile('static', 'public/ghost.css', 'text/css', constants.ONE_HOUR_S));

ghost/core/core/shared/config/defaults.json

@@ -117,6 +117,9 @@
         "sitemap": {
             "maxAge": 3600
         },
+        "sitemapXSL": {
+            "maxAge": 86400
+        },
         "robotstxt": {
             "maxAge": 3600000
         },