Wagtail-react-streamfield

- Add vendor prefixes to the project’s stylesheet, to ensure styles are cross-browser-compatible (`user-select`, `appearance`, and `backface-visibility`).
- Fix Django 3.0 compatibility issue due to removed `django.utils.six` ([55](https://github.com/wagtail/wagtail-react-streamfield/issues/55), [56](https://github.com/wagtail/wagtail-react-streamfield/pull/56)). Thanks to [lennsa](https://github.com/lennsa), [colinappnovation](https://github.com/colinappnovation), and [zerolab](https://github.com/zerolab).

Fixes validation errors in nested StructBlocks or ListBlocks

Fixes the broken JavaScript from TableBlock

Fixes a packaging issue due to a setuptools bug (it doesn’t clean its
workspace before rebuilding, leading to extra unwanted files in the package)

Fixes an issue with collapsible `FieldPanel`
(nothing related to StreamFields) no longer working properly due to an
overridden JavaScript file lagging behind the official version

Fixes another issue with radio buttons (the serialization was not right)