Convert more admin fields to functional components

[emteknetnz]

There are react class component(s) that needs to be converted to react functional component(s).

• CheckboxSetField.js
• CompositeField.js
• FieldGroup.js
• FieldHolder.js
• FormAction.js
• HiddenField.js

Note there are more components than this to convert, limiting number converted at a time to make peer review more manageable.

Acceptance criteria

• Component(s) are converted to a functional component using hooks
• Jest test coverage is increased
• All existing functionality is preserved
• Legacy class components are retained (if applicable i.e. is admin form field component)

PRs

• ENH Convert more admin fields to functional components #2149
• ENH Convert HistoricElementView to functional component silverstripe-elemental#1434
• ENH Add CompositeField and FieldGroup to externals.js webpack-config#93