some updates to page loading performance and general performance.

added a ServiceWorker which caches stuff

js/post/sw.js

@@ -0,0 +1,32 @@
+const CACHE_NAME = 'offline-v1';
+const ASSETS = [
+  '/css/main.css',
+  '/js/shared/theme.js',
+  '/js/post/download.js',
+  '/js/post/lazyimg.js',
+  '/package/js/prism.min.js',
+  '/package/js/prism-python.min.js',
+  '/package/js/mathjax.js',
+  '/css/icons/back.webp',
+  '/css/icons/written.webp',
+  '/css/icons/date.webp',
+  '/css/icons/magnifier.webp',
+  '/css/icons/save.webp',
+  '/css/icons/script.webp'
+];
+
+self.addEventListener('install', e => {
+  e.waitUntil(caches.open(CACHE_NAME).then(c => c.addAll(ASSETS)));
+});
+
+self.addEventListener('fetch', e => {
+  e.respondWith(
+    caches.match(e.request).then(res => res || fetch(e.request).then(resp => {
+      if (e.request.url.startsWith(self.location.origin)) {
+        const copy = resp.clone();
+        caches.open(CACHE_NAME).then(c => c.put(e.request, copy));
+      }
+      return resp;
+    }))
+  );
+});