MDL-69422 mod_data: Export form accessibility fixes
* Set appropriate heading element IDs
* Replace usages of deprecated HTML tag acronym with abbr instead.
* Add a label for the export fields fieldset.
* Move the CSV delimiter list outside the radio group.
* Fix div under a label element and duplicate labels
for the checkboxes of the fields to be exported by showing the field
name and type together.
* Group the fields to be exported as a fieldset.
* Move unsupported fields into a separate list.
* Move export options into its own section and group the options as a
fieldset.
