app directory migration

[Entkenntnis]

This PR will migrate all of pages into app. It's faster to do it in one go and many things are already working nicely.

list of current to-dos:

• review metadata handling: replace headtags with generateMetaData (it's moved out because head tags should be available before streaming, so it has to be fairly independant of page data, which we are not doing consistently)
• bundle size analyzer not showing app-dir pages - Support app directory hashicorp/nextjs-bundle-analysis#42
• every page has to pass locale and instanceData to FrontendClientBase -> search for abstraction
• get translations on server is a bit inconvenient because lang is only accessible in page.tsx, maybe use cache
• navigation through router.push not working in vercel deployments (localhost is fine) (rewrite is not picked up ...) host rewrites client side navigation works all fine in dev mode, but stop working after deploy to Vercel (All works fine before this week) vercel/next.js#54783
• old router is used here and there
• migrate api routes to route handlers
• restore progress bar (replace Router events)
• fix injections
• access pathname is difficult for meta data How to access `pathname` in `generateMetadata`? vercel/next.js#50189

preview: https://frontend-git-2823-another-attempt-at-adopting-app-234955-serlo.vercel.app/1553

[vercel]

The latest updates on your projects. Learn more about Vercel for Git ↗︎

Name	Status	Preview	Updated (UTC)
frontend	✅ Ready (Inspect)	Visit Preview	Aug 31, 2023 9:46am

[github-actions]

📦 Next.js Bundle Analysis for @serlo/frontend

This analysis was generated by the Next.js Bundle Analysis action. 🤖

🎉 Global Bundle Size Decreased

Page	Size (compressed)
global	75.54 KB (🟢 -22 KB)

Details

The global bundle is the javascript bundle that loads alongside every page. It is in its own category because its impact is much higher - an increase to its size means that every page on your website loads slower, and a decrease means every page loads faster.

Any third party scripts you have added directly to your app using the <script> tag are not accounted for in this analysis

If you want further insight into what is behind the changes, give @next/bundle-analyzer a try!

One Page Changed Size

The following page changed size from the code in this PR compared to its base branch:

Page	Size (compressed)	First Load
/_error	230 B (🟢 -1 B)	75.76 KB

Details

Only the gzipped size is provided here based on an expert tip.

First Load is the size of the global bundle plus the bundle for the individual page. If a user were to show up to your website and land on a given page, the first load size represents the amount of javascript that user would need to download. If next/link is used, subsequent page loads would only need to download that page's bundle (the number in the "Size" column), since the global bundle has already been downloaded.

Any third party scripts you have added directly to your app using the <script> tag are not accounted for in this analysis

Next to the size is how much the size has increased or decreased compared with the base branch of this PR. If this percentage has increased by 20% or more, there will be a red status indicator applied, indicating that special attention should be given to this.

[Entkenntnis]

@elbotho

Approach here: Make language static. You can still easily change it to another language if you want to develop for it. This way, no middleware is needed (and this makes the mental model a lot easier)

[Entkenntnis]

This attempt got quite far, but due to some remaining bugs and challenges (see description), we will postpone this for some more months.