Improve a11y of form fields

[BurntimeX]

• [ ] Use aria-invalid=true to flag invalid form field inputs
• [ ] Use aria-errormessage to identify the element that contains the error message
• [ ] Ensure that the label does not refer to a non-existent element.
• [ ] Form fields consisting of multiple inputs (e.g. radio buttons) should be grouped together in a role=groupelement or in a fieldset.

ref https://github.com/WoltLab/WCF/issues/6519 ref https://www.woltlab.com/community/thread/313994-fehlervalidierung-verbessern-wcag-guideline-3-3/ ref https://www.woltlab.com/community/thread/312190-bfsg-aa-konformit%C3%A4t-dt-oder-dd-element-nicht-von-dl-umgeben/