Make wagtail streamforms compatible with wagtail 4.1.1

[fabFrankDouwes]

• bump version number
• add note to readme regarding older versions.
• StreamFieldPanel to FieldPanel conversion (required for wagtail 3+).
• add use_json_field to FormFieldsStreamField (required for wagtail 3+).
• tweak FormChooserBlock to work like it did in wagtail 2.15 (wagtail is moving towards chooser overlays).

quick note:

• by applying these changes we're dropping compatibility for 2.x, the readme has been updated to reflect this.
• this clearly needs further testing, the basic features (creation of forms / the form block / submitting forms via email all work), none of the advanced features have been tested.
• this has not been tested with wagtail 3.x (as it will never get the LTS status).

[leongraumans]

Hi @fabFrankDouwes,

My colleague has updated the package for Django 4.0 and Wagtail 4.0. Could you rebase your branch and update your PR where necessary? I'll then try to review & release it asap :)

Thanks!

[leongraumans]

Hi @fabFrankDouwes ,

Please see the new(er) release: https://github.com/labd/wagtailstreamforms/releases/tag/3.21.0 which adds support for Wagtail 4.1.x

Compared to that release, I see your PR has some extra logic like the get_form_state method. If you'd still like to merge those changes, I'd kindly ask you to update your PR.
Also, a version number in the init is not neccessary, as I will provide one with the next release.

Thanks, and have a nice weekend :)