Wagtail-react-streamfield

- Dropped Wagtail < 2.6 support
- Removes layouts, only a single layout exists now, a mix of the best parts of
the former `SIMPLE` of `COLLAPSIBLE`
- Adds `closed` attribute in the `Meta` of blocks to set by block type
whether they should be closed on page editor load (defaults to `False`)
- Adds support for collapsible nested struct blocks
- Speeds up server-side page editor load
- Fixes two issues with radio buttons
- Fixes an issue with missing IDs in StreamBlocks
(including the root StreamBlock)
- Fixes callable default values
- Fixes the dynamic title of the children of a `ListBlock`
- Fixes minor CSS details
- Renamed `Block.get_definition()` to a `Block.definition` cached property
(overriding it with a simple `property` works, but it will be faster
with `cached_property` from `django.utils.functional`)

- Dropped Wagtail 2.2 & 2.3 support, but it may work perfectly with both these
versions, it was not tested on these
- Moves the plusses between blocks (they were in the left gutter)
- Adds the duplicate icon that was missing from the MANIFEST.in
- Fixes `COLLAPSIBLE` block content previews on non-struct blocks
- Rewrites the SCSS using BEM to avoid clashes with external CSS
- Countless minor visual fixes
- Major rewrite to improve code quality and extensibility

- Fixes a bug was introduced in 1.1.0 when validation fails while saving
- Removes a rectangle visible on Safari due to a weird unicode character
added by accident

- Automatically collapses blocks on small/mobile devices
- Increases the size of block type selection buttons while making the labels
uppercase
- Adds support for help text in StreamBlock, ListBlock & StructBlock
- Adds support for default values in StreamBlock, ListBlock & StructBlock
- Adds support for non-block errors in non-root StreamBlock, ListBlock
& StructBlock
- Applies default values to missing sub-blocks of already saved StructBlocks
- Fixes the remaining edge cases found by updating tests upstream,
in https://github.com/wagtail/wagtail/pull/4942

Fixes a formatting issue with `AdminDateInput` & `AdminDateTimeInput`
due to Wagtail’s custom formatting
(`WAGTAIL_DATE_FORMAT` & `WAGTAIL_DATETIME_FORMAT`)

- Adds a real duplicate icon
- Fix a recently introduced bug raising a 500 error when saving a page
with validation errors