* Implement MultipleChoiceBlock (squashed commits from #5592)
* Omit widget from frozen kwargs
* Rename get_callable_choices to indicate it is an internal method
* Add release notes for MultipleChoiceBlock
Currently it is possible to pass the target attribute to a button
created using ModelAdmin's ButtonHelper framework. This allows you to
generate button links like <a ... target="_blank">.
For example, if adding a new button and modeling it after the existing
edit_button code [0], you can add {'target': "_blank"} to the returned
dict and it'll get passed to the template when the button is rendered.
To be consistent with PR 4844, and to be consistent with what seems to
be the best practice ([1], [2]), we should also support passing the rel
attribute, which would allow for creation of button links like <a ...
target="_blank" rel="noopener noreferrer">.
[0] 5e2f50403b/wagtail/contrib/modeladmin/helpers/button.py (L61-L73)
[1] https://developers.google.com/web/tools/lighthouse/audits/noopener
[2] https://www.jitbit.com/alexblog/256-targetblank---the-most-underestimated-vulnerability-ever/
