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/WebPlatform.js

@@ -142,6 +142,10 @@ export default class WebPlatform extends VectorBasePlatform {
         setInterval(this.pollForUpdate.bind(this), POKE_RATE_MS);
     }
 
+    async canSelfUpdate(): boolean {
+        return true;
+    }
+
     pollForUpdate() {
         return this._getVersion().then((ver) => {
             if (this.runningVersion === null) {