Made Content API 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/server/web/api/endpoints/content/app.js

@@ -3,6 +3,7 @@ const boolParser = require('express-query-boolean');
 const bodyParser = require('body-parser');
 const express = require('../../../../../shared/express');
 const sentry = require('../../../../../shared/sentry');
+const config = require('../../../../../shared/config');
 const shared = require('../../../shared');
 const routes = require('./routes');
 const errorHandler = require('@tryghost/mw-error-handler');
@@ -22,7 +23,7 @@ module.exports = function setupApiApp() {
 
     // Content API should allow public caching
     apiApp.use(shared.middleware.cacheControl('public', {
-        maxAge: 0
+        maxAge: config.get('caching:contentAPI:maxAge')
     }));
 
     // Routing

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

@@ -131,6 +131,9 @@
         },
         "cors": {
             "maxAge": 86400
+        },
+        "contentAPI": {
+            "maxAge": 0
         }
     },
     "imageOptimization": {