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).

webpack.config.js

@@ -108,11 +108,6 @@ module.exports = {
             "matrix-js-sdk": path.resolve('./node_modules/matrix-js-sdk'),
         },
     },
-    externals: {
-        // Don't try to bundle electron: leave it as a commonjs dependency
-        // (the 'commonjs' here means it will output a 'require')
-        "electron": "commonjs electron",
-    },
     plugins: [
         new webpack.DefinePlugin({
             'process.env': {