[TASK] Switch from jQuery to vanilla JavaScript

[rintisch]

This is WIP because some points are not final:

• There are two spots in the JS marked with "Todo"
• The JavaScript for the form finisher is not included anywhere. Therefore it is not tested as well.

This ticket solves #333.

This fixes also https://docs.typo3.org/p/extcode/cart/8.6/en-us/Changelog/8.5/Important-404-ChangeCheckboxValueInOrderForm.html for the multistep checkout where this adaption was not done.