Harmonize required input fields and add a11y

Follow-up of #3994. Some of the e-input fields feature a red asterisk when they are marked as required, some others don't. All of them should be treated the same way.
Also, make sure the required status is communicated understandably for screen readers etc., according to https://www.accessibility-developer-guide.com/examples/forms/required/ (and ideally testing with a screen reader).