Make sure that manifest URL is using base url while checking

[TimoPtr]

Summary

While testing some error screen I found out that the manifest check was wrong because it was using the full URL given, where the manifest.json should only be checked on the base URL otherwise it doesn't work.

Checklist

• New or updated tests have been added to cover the changes following the testing guidelines.
• The code follows the project's code style and best_practices.
• The changes have been thoroughly tested, and edge cases have been considered.
• Changes are backward compatible whenever feasible. Any breaking changes are documented in the changelog for users and/or in the code for developers depending on the relevance.

Any other notes

I renamed the function, added some docs and test.