Plone.volto

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- Fix deprecated import of isDefaultPage
[pbauer]

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- Fix Multilingual profile, revert to use draftJS (until slate is integrated into Volto)
[sneridagh]

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- Bring back the draftJS as default, until Slate is integrated in full in Volto
[sneridagh]

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Breaking:

- Use Slate blocks for the default pages
[sneridagh]

- Add ``volto.blocks`` behavior to Plone Site GS types info.
[sneridagh]

- Remove ``Collection`` from types in GS types info.
[sneridagh]

- Remove ``plone.richtext`` behavior from Plone Site, Document, News Item, Events
[sneridagh]

- Add blocks behavior on Event and NewsItem
[nzambello]

- Add preview_image to Event
[nzambello]

Internal:

- Code cleanup, remove some outdated and unused helpers in ``setuptools.py``
[sneridagh]

- Workaround a test fixture isolation issue with the `IVoltoSettings.frontend_domain`
setting.
[rpatterson]

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Breaking:

- Remove c.folderishtypes dependency

New Feature:

- Add new field in the coresandbox: not constrained by vocabulary field but the vocabulary defined in the widget.
[sneridagh]

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- Computed copyright dates for content demo pages
[sneridagh]