Add support for general `FieldPanel` with Wagtail 3.0+

[zerolab]

References:

• https://docs.wagtail.org/en/latest/releases/3.0.html#removal-of-special-purpose-field-panel-types
• https://docs.wagtail.org/en/latest/releases/3.0.html#api-changes-to-panels-edithandlers
• https://docs.wagtail.org/en/latest/extending/forms.html#defining-admin-form-widgets

Where possible, third-party packages that implement their own field panel types should be updated to allow using a plain FieldPanel instead, in line with Wagtail dropping its own special-purpose field panel types such as StreamFieldPanel and ImageChooserPanel. The steps for doing this will depend on the package’s functionality, but in general:

• If the panel sets a custom template, your code should instead define a Widget class that produces your desired HTML rendering.
• If the panel provides a widget_overrides method, your code should instead call register_form_field_override so that the desired widget is always selected for the relevant model field type.
• If the panel provides a get_comparison_class method, your code should instead call wagtail.admin.compare.register_comparison_class to register the comparison class against the relevant model field type.

[zerolab]

Turns out.. not needed at all