Form templates: Improve WCAG AA level compliance

Description

This story makes forms more user-friendly and accessible, specifically level AA of WCAG compliance. It fixes the following problems.

Checkbox and radio buttons

• You don't get a field set enclosing the elements.
• The heading is a <span> instead of a <legend>.

Attributes

• For usability and accessibility reasons, try to mark HTML5 attributes, such as the required attribute.
• Ability to set types on input.