[image file view] rewrote it with react (#2983)

* [image file view] rewrote it with react

* [image file view] modification

* [image file view] modified code indentation & etc.

frontend/src/components/file-view/file-view-tip.js

@@ -0,0 +1,26 @@
+import React from 'react';
+import { gettext } from '../../utils/constants';
+
+const { err } = window.app.pageOptions;
+
+class FileViewTip extends React.Component {
+
+  render() {
+    let errorMsg;
+    if (err == 'File preview unsupported') {
+      errorMsg = <p>{gettext('Online view is not applicable to this file format')}</p>;
+    } else {
+      errorMsg = <p className="error">{err}</p>;
+    }   
+    return (
+      <div className="file-view-content flex-1">
+        <div className="file-view-tip">
+          {errorMsg}
+          <a href="?dl=1" className="btn btn-secondary">{gettext('Download')}</a>
+        </div>
+      </div>
+    );  
+  }
+}
+
+export default FileViewTip;