Refactor resizeImage method (#7236)

- Use URL.createObjectURL (replace from FileReader)
- Use HTMLCanvasElement.prototype.toBlob
  (replace from HTMLCanvasElement.prototype.toDataURL)
- Use Promise (replace callback interface)

app/javascript/mastodon/utils/base64.js

@@ -0,0 +1,10 @@
+export const decode = base64 => {
+  const rawData = window.atob(base64);
+  const outputArray = new Uint8Array(rawData.length);
+
+  for (let i = 0; i < rawData.length; ++i) {
+    outputArray[i] = rawData.charCodeAt(i);
+  }
+
+  return outputArray;
+};