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/VectorBasePlatform.js

@@ -3,6 +3,7 @@
 /*
 Copyright 2016 Aviral Dasgupta
 Copyright 2016 OpenMarket Ltd
+Copyright 2018 New Vector Ltd
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
@@ -45,7 +46,6 @@ export default class VectorBasePlatform extends BasePlatform {
         this.favicon = new Favico({animation: 'none'});
         this.showUpdateCheck = false;
         this._updateFavicon();
-        this.updatable = true;
 
         this.startUpdateCheck = this.startUpdateCheck.bind(this);
         this.stopUpdateCheck = this.stopUpdateCheck.bind(this);
@@ -88,6 +88,19 @@ export default class VectorBasePlatform extends BasePlatform {
         this._updateFavicon();
     }
 
+    supportsAutoLaunch() {
+        return false;
+    }
+
+    // XXX: Surely this should be a setting like any other?
+    async getAutoLaunchEnabled() {
+        return false;
+    }
+
+    async setAutoLaunchEnabled(enabled) {
+        throw new Error("Unimplemented");
+    }
+
     /**
      * Begin update polling, if applicable
      */
@@ -97,8 +110,8 @@ export default class VectorBasePlatform extends BasePlatform {
     /**
      * Whether we can call checkForUpdate on this platform build
      */
-    canSelfUpdate(): boolean {
-        return this.updatable;
+    async canSelfUpdate(): boolean {
+        return false;
     }
 
     startUpdateCheck() {