Drop specialised panels in favour of a generic ViewSet

[zerolab]

https://docs.wagtail.org/en/stable/extending/generic_views.html

• Would fix Add support for general FieldPanel with Wagtail 3.0+ #155 and AdminChooser needs to be replaced with BaseChooser #189
• Would drop the specialised Audio/Video choosers or the ability to pass media_type to the chooser <-- this is the most controversial bit
• Would have less code (JS, HTML)

What do folk think?