Saving playground code state (#1917)

The embedded Playground's code reset when you navigate between slides.
This has caused problems: if people navigate away from a slide to look
something up, they've suddenly lost their work.

theme/save-playgrounds.js

@@ -0,0 +1,46 @@
+(function savePlaygrounds() {
+  function setCodeToPlayground() {
+    var codes = JSON.parse(
+      localStorage.getItem(`${window.location.href}₹code`)
+    );
+    if (codes) {
+      var i = 0;
+      Array.from(document.querySelectorAll(".playground")).forEach(function (
+        pre_block
+      ) {
+        let code_block = pre_block.querySelector("code");
+        let editor = window.ace.edit(code_block);
+        editor.setValue(codes[i]);
+        editor.clearSelection();
+        i += 1;
+      });
+    }
+  }
+  function getCodeFromPlayground() {
+    var codes = [];
+    Array.from(document.querySelectorAll(".playground")).forEach(function (
+      pre_block
+    ) {
+      let code_block = pre_block.querySelector("code");
+      let editor = window.ace.edit(code_block);
+      let code = editor.getValue();
+      codes.push(code);
+    });
+    localStorage.setItem(`${window.location.href}₹code`, JSON.stringify(codes));
+  }
+  setCodeToPlayground();
+  addEventListener("pagehide", getCodeFromPlayground);
+})();
+
+function resetPlaygroundsClicked() {
+  let keys = [];
+  for (var i = 0, len = localStorage.length; i < len; i++) {
+    if (localStorage.key(i).includes("₹code")) {
+      keys.push(localStorage.key(i));
+    }
+  }
+  for (let j = 0; j < keys.length; j++) {
+    localStorage.removeItem(keys[j]);
+  }
+}
+window.resetPlaygroundsClicked = resetPlaygroundsClicked;