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/__tests__/base64-test.js

@@ -0,0 +1,10 @@
+import * as base64 from '../base64';
+
+describe('base64', () => {
+  describe('decode', () => {
+    it('returns a uint8 array', () => {
+      const arr = base64.decode('dGVzdA==');
+      expect(arr).toEqual(new Uint8Array([116, 101, 115, 116]));
+    });
+  });
+});