Start the switch to prop-types

As https://facebook.github.io/react/docs/typechecking-with-proptypes.html
says, React.PropTypes have moved into the npm package prop-types since
React v15.5. I've already failed to update certain devDependencies
because they mandate the use of prop-types so I'd rather we (gradually
at least) move to prop-types rather than face a lot of work later on.

package.json

     "lodash": "4.17.4",
     "nuclear-js": "1.4.0",
     "postis": "2.2.0",
+    "prop-types": "15.5.10",
     "react": "15.6.1",
     "react-dom": "15.6.1",
     "react-i18next": "4.8.0",

react/features/app/components/AbstractApp.js

+import PropTypes from 'prop-types';
 import React, { Component } from 'react';
 import { I18nextProvider } from 'react-i18next';
 import { Provider } from 'react-redux';
          * The default URL {@code AbstractApp} is to open when not in any
          * conference/room.
          */
-        defaultURL: React.PropTypes.string,
+        defaultURL: PropTypes.string,
 
         /**
          * (Optional) redux store for this app.
          */
-        store: React.PropTypes.object,
+        store: PropTypes.object,
 
         /**
          * The URL, if any, with which the app was launched.
          */
-        url: React.PropTypes.oneOfType([
-            React.PropTypes.object,
-            React.PropTypes.string
+        url: PropTypes.oneOfType([
+            PropTypes.object,
+            PropTypes.string
         ])
     };