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 #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/