The automatic upgrade failed: kantord/LibreLingo#786

The task is to fix the issues and make the upgrade pass.

This should be a good first issue as it will not require deep knowledge of the project, but keep in mind that there's no guarantees that updating webpack will immediately work, because it's a fresh release.

Official blog on version 5: https://webpack.js.org

Right now when the user logs out, the Decks widget looses access to the list of decks, making Firebase trigger permission denied events. This results in issue churn:

• https://sentry.io/share/issue/c903ffaafb1d4f09903b614cfc6bdba5/
• https://sentry.io/share/issue/e687924953344d13960ac45ba0794877/
• https://sentry.io/share/issue/a69efe18a23945cbb5cbfe3bf7072340/

We should not receive except