Accessibility labelling for forms

[jace]

Forms need aria labels for accessibility:

1. Rename ref_id to form_dom_id to indicate that this is an identifier in the DOM
2. All labels, descriptions and error messages accompanying a field should have id tags in the form {{ form_dom_id }}-label, -description, -error-0, etc. Since there could be multiple errors, they need to be indexed.
3. The fields themselves need two additional ARIA tags:
   1. aria-describedby which has all of the above ids, separated by spaces
   2. aria-invalid, whose value is {% if field.errors %}true{% else %}false{% endif %}