Change PureComponent to ImmutablePureComponent (#3460)

* Change PureComponent to ImmutablePureComponent

Change PureComponent to ImmutablePureComponent in AttachmentList.

* rm React.

app/javascript/mastodon/components/attachment_list.js

 import React from 'react';
 import ImmutablePropTypes from 'react-immutable-proptypes';
+import ImmutablePureComponent from 'react-immutable-pure-component';
 
 const filename = url => url.split('/').pop().split('#')[0].split('?')[0];
 
-class AttachmentList extends React.PureComponent {
+class AttachmentList extends ImmutablePureComponent {
 
   static propTypes = {
     media: ImmutablePropTypes.list.isRequired,