Make deletion of scenarios possible

[annawendler]

Description

At the moment, we can't delete scenarios using the delete_scenario command. When changing on_delete=models.RESTRICT to on_delete=models.CASCADE this is possible.

Checklist

I, the author of this PR checked the following requirements for good software quality:

• The code is properly formatted (I ran the formatter)
• The code is written with our software quality standards (I ran the linter)
• The code is written using our code style
• Extensive in source documentation has been added
• Unit and/or integration tests have been added
• All texts have been internationalized with at least the following languages:
  • English
  • German
• I tried addressing all new accessibility problems displayed in the console and documented if they can't be fixed
• I attached performance measurements to prevent performance degradation
• I added the changes to the next release section of the changelog

I, the reviewer checked the following things:

• I ran the software once and tried all new and related functionality to this PR
• I looked at all new and changed lines of code and commented on possible problems
• I read the added documentation and checked if it is understandable and clear
• I checked the added tests for completeness
• I checked the internationalized strings for spelling errors
• I checked the performance metrics for problems or unexplained degradation
• I checked that the changes are noted in the changelog