[CONTENT] Remove start codes

[Felienne]

Fixes #4906 by removing all start_codes and associated test code and validation.

How to test
Verify that everything works without using start_codes

[ghost]

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Legend

[Felienne]

Dropping the current Weblate stats from the readme here because I want to see the impact on the percentages after merge :)

[Felienne]

Hi @SabinaChita!

This is a small change in content, but it will have quite an impact on teaching, I suspect! Here are some ideas to place in the newsletter, feel free to adapt where needed, the headers are more for you that to be placed in the newsletter like this! The screenshots are also for you, I can imagine we will make other ones for the newsletter.

How it used to work
All levels used to have a start_code, if you go to a new adventure, code would be shown in the editor:

How it works now
There are no more start_codes, so if you go to a new tab now, where you have never created something, the editor box will be empty:

Why
Since we have the exercises now, we want kids to start from the exercise code, run that and follow the exercise. So the start code is more a distraction than helpful.

Potential impact on teaching
I suspect that a lot of kids in the current start just run the start code and are happy everything is working. Teachers might have to work a little bit harder to get kids to get started, but this will be worth it hopefully since it focuses their attention on the exercise.

[mergify]

Thank you for contributing! Your pull request is now going on the merge train (choo choo! Do not click update from main anymore, and be sure to allow changes to be pushed to your fork).