brouillon: redirect to sign-in when disconnected

There are two cases where the draft auto-save might fail because the
user is no longer authenticated:

- The user signed-out in another tab,
- The brower quit and re-opened, so the Session cookie expired.

In both cases, the auto-save will never succeed until the user
authenticates again, so displaying a "Retry" button is cruel.

Moreover, in plus of all auto-save requests failing with a small error,
the actual hard failure only occurs after filling all the form and
trying to submit it. Then the user is redirected to the sign-in page –
but all their changes are lost.

Instead, we now redirect to the sign-in page on the first 401 error
during the auto-save, let the user sign-in, and then redirect back to
the form.

app/javascript/new_design/dossiers/auto-save.js

@@ -62,6 +62,15 @@ addEventListener('autosave:end', () => {
 });
 
 addEventListener('autosave:error', (event) => {
+  let error = event.detail;
+
+  if (error.xhr.status == 401) {
+    // If we are unauthenticated, reload the page using a GET request.
+    // This will allow Devise to properly redirect us to sign-in, and then back to this page.
+    document.location.reload();
+    return;
+  }
+
   enable(document.querySelector('button.autosave-retry'));
   setState('failed');
   logError(event.detail);