Enforce sandbox on all spawned BrowserWindow objects

The docs (https://www.atom.pe/docs/api/sandbox-option/) say we should be using the browser-native `window.open` implementation, but in practice that appears very much false. Electron, no matter our set of options, appears to always make a hit to the ipcRenderer with `window.open` calls, causing the calling code to explode due to the sandbox making that impossible.

By using `app.enableSandbox()`, it puts the sandbox in place over all BrowserWindow objects, including the temporary ones which empirically are being created for `window.open`. We do not need to specify `sandbox: true` to the BrowserWindow with this approach, though uncommenting and therefore reintroducing the flag causes our lovely ipcRenderer error again.

As far as I can tell, the sandbox does actually get applied to the window though the fact that `sandbox: true` still does things despite the docs saying otherwise leaves me a bit uncomfortable.

Fixes https://github.com/vector-im/riot-web/issues/13719

src/electron-main.js

@@ -615,6 +615,17 @@ protocol.registerSchemesAsPrivileged([{
     },
 }]);
 
+// Turn the sandbox on for *all* windows we might generate. Doing this means we don't
+// have to specify a `sandbox: true` to each BrowserWindow.
+//
+// This also fixes an issue with window.open where if we only specified the sandbox
+// on the main window we'd run into cryptic "ipc_renderer be broke" errors. Turns out
+// it's trying to jump the sandbox and make some calls into electron, which it can't
+// do when half of it is sandboxed. By turning on the sandbox for everything, the new
+// window (no matter how temporary it may be) is also sandboxed, allowing for a clean
+// transition into the user's browser.
+app.enableSandbox();
+
 app.on('ready', async () => {
     try {
         await setupGlobals();
@@ -725,7 +736,7 @@ app.on('ready', async () => {
         webPreferences: {
             preload: preloadScript,
             nodeIntegration: false,
-            sandbox: true,
+            //sandbox: true, // We enable sandboxing from app.enableSandbox() above
             enableRemoteModule: false,
             // We don't use this: it's useful for the preload script to
             // share a context with the main page so we can give select