[Glitch] Use inert polyfill

Port 84e3b5a358 to glitch-soc

Signed-off-by: Thibaut Girka <thib@sitedethib.com>

app/javascript/flavours/glitch/components/modal_root.js

@@ -1,5 +1,6 @@
 import React from 'react';
 import PropTypes from 'prop-types';
+import 'wicg-inert';
 import createHistory from 'history/createBrowserHistory';
 
 export default class ModalRoot extends React.PureComponent {