Run the Desktop app in a sandbox

* Turn off node integration in the electron renderer process
 * Enable the chromium sandbox to put the renderer into its own process
 * Expose just the ipc module with a preload script
 * Introduce a little IPC call wrapper so we can call into the
   renderer process and await on the result.
 * Use this in a bunch of places we previously used direct calls
   to electron modules.
 * Convert other uses of node, eg. use of process to derive the
   platform (just look at the user agent)
 * Strip out the desktopCapturer integration which doesn't appear
   to have ever worked (probably best to just wait until
   getDisplayMedia() is available in chrome at this point:
   https://github.com/vector-im/riot-web/issues/4880).

src/vector/platform/index.js

@@ -1,29 +0,0 @@
-// @flow
-
-/*
-Copyright 2016 Aviral Dasgupta
-Copyright 2016 OpenMarket Ltd
-
-Licensed under the Apache License, Version 2.0 (the "License");
-you may not use this file except in compliance with the License.
-You may obtain a copy of the License at
-
-    http://www.apache.org/licenses/LICENSE-2.0
-
-Unless required by applicable law or agreed to in writing, software
-distributed under the License is distributed on an "AS IS" BASIS,
-WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-See the License for the specific language governing permissions and
-limitations under the License.
-*/
-
-let Platform = null;
-
-if (window && window.process && window.process && window.process.type === 'renderer') {
-    // we're running inside electron
-    Platform = require('./ElectronPlatform');
-} else {
-    Platform = require('./WebPlatform');
-}
-
-export default Platform;