Use ES Class Fields & Static Properties (#3008)

Use ES Class Fields & Static Properties (currently stage 2) for improve class outlook.

Added babel-plugin-transform-class-properties as a Babel plugin.

app/javascript/mastodon/features/compose/components/upload_progress.js

@@ -5,6 +5,11 @@ import { FormattedMessage } from 'react-intl';
 
 class UploadProgress extends React.PureComponent {
 
+  static propTypes = {
+    active: PropTypes.bool,
+    progress: PropTypes.number
+  };
+
   render () {
     const { active, progress } = this.props;
 
@@ -35,9 +40,4 @@ class UploadProgress extends React.PureComponent {
 
 }
 
-UploadProgress.propTypes = {
-  active: PropTypes.bool,
-  progress: PropTypes.number
-};
-
 export default UploadProgress;