Generate production sourcemaps

[codemonkey800]

Not having source maps in production impacts our Best Practices Lighthouse score:

We should look into how we can generate source maps for the frontend. I've tried following the instructions from the Next.js docs, but for some reason source maps were still not being generated.

This might be an issue with the current version of Next.js we're using, so it might be solved by #610. However, this PR has been open for a while because of failing E2E tests, so it might be good to give it another try once all of the E2E test PRs are merged

[codemonkey800]

Upgrade Next.js spike: #967