feat(editor): show user a warning in test environments that content might disappear

[LarsTheGlidingSquirrel]

Background: Some people were excited to use the editor and started to create content they want to keep even in the test environments. But for the devs this makes it more complicated.

Change: All test environments show a warning label above the editor.

@elbotho @GregorZupan Any wishes in terms of design / wording? Would you prefer to not have something like this?

[vercel]

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Name	Status	Preview	Updated (UTC)
frontend	✅ Ready (Inspect)	Visit Preview	Oct 23, 2024 1:50pm

[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	104.58 KB (-1 B)

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!

Seven Pages Changed Size

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

Page	Size (compressed)	First Load
/___editor_preview	529.62 KB (🟡 +124 B)	634.2 KB
/entity/create/[type]/[taxonomyId]	549.69 KB (🟡 +124 B)	654.27 KB
/entity/repository/add-revision/[...id]	549.43 KB (🟡 +124 B)	654.01 KB
/page/create	549.42 KB (🟡 +124 B)	654 KB
/taxonomy/term/create/[parent_id]/[id]	549.17 KB (🟡 +124 B)	653.75 KB
/taxonomy/term/update/[id]	549.12 KB (🟡 +124 B)	653.69 KB
/user/settings	551.11 KB (🟡 +124 B)	655.69 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.

[elbotho]

Two small things.
I like it overall, thank you.
(Maybe I will make it more prominent at some point)