Made Admin assets aggressively cacheable

closes https://github.com/TryGhost/Toolbox/issues/372

- The admin assets are served with a unique hash depending on the build with a year-long "max-age" value in the response cache-control header. The client browsers still do send 'If-None-Match' requests when there is a hard-refresh on the client side. There's no need for 'If-None-Match' requests though!
- With 'immutable' value in the cache-control header, the browser caches are treating responses as "hard-fresh" without sending redundant requests.
- For more about 'immutable' value read https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Cache-Control#immutable

ghost/core/core/server/web/admin/app.js

@@ -17,9 +17,15 @@ module.exports = function setupAdminApp() {
     // Admin assets
     // @TODO ensure this gets a local 404 error handler
     const configMaxAge = config.get('caching:admin:maxAge');
+    // @NOTE: when we start working on HTTP/3 optimizations the immutable headers
+    //        produced below should be split into separate 'Cache-Control' entry.
+    //        For reference see: https://developer.mozilla.org/en-US/docs/Web/HTTP/Caching#validation_2
     adminApp.use('/assets', serveStatic(
-        path.join(config.get('paths').adminAssets, 'assets'),
-        {maxAge: (configMaxAge || configMaxAge === 0) ? configMaxAge : constants.ONE_YEAR_MS, fallthrough: false}
+        path.join(config.get('paths').adminAssets, 'assets'), {
+            maxAge: (configMaxAge || configMaxAge === 0) ? configMaxAge : constants.ONE_YEAR_MS,
+            immutable: true, 
+            fallthrough: false
+        }
     ));
 
     adminApp.use('/auth-frame', serveStatic(

ghost/core/test/e2e-server/admin.test.js

@@ -38,7 +38,7 @@ describe('Admin Routing', function () {
 
         it('should retrieve built assets', async function () {
             await request.get('/ghost/assets/vendor.js')
-                .expect('Cache-Control', testUtils.cacheRules.year)
+                .expect('Cache-Control', testUtils.cacheRules.yearImmutable)
                 .expect(200)
                 .expect(assertCorrectHeaders);
         });

ghost/core/test/utils/fixtures/cache-rules.js

@@ -3,5 +3,6 @@ module.exports = {
     hour: 'public, max-age=' + 3600,
     day: 'public, max-age=' + 86400,
     year: 'public, max-age=' + 31536000,
+    yearImmutable: 'public, max-age=' + 31536000 + ', immutable',
     private: 'no-cache, private, no-store, must-revalidate, max-stale=0, post-check=0, pre-check=0'
 };