Improve page chooser (widgets.AdminChooser) when copying a page

[msochel]

Is your proposal related to a problem?

When a user is in the form to copy a page (CopyForm), some pages appear as available destinations when in fact it is not a possible destination due to the hierarchy of the page to copy. Currently playing with the bakarydemo project, I am copying a
location type page and the widgets.AdminPageChooser visually show and lets select other types of pages as possible destinations. At the end of the flow, it throws an error notification that may not be too clear.

Describe the solution you'd like

I would like the page chooser to gray out the page types that do not follow the hierarchy.

My suggestion is to pass the target_models parameter to widgets.AdminPageChooser on CopyForm, which allows us to specify the types of pages that can only be selectable. In this case, I would suggest passing as the value of this parameter the value returned by the function allowed_parent_page_models

self.fields["new_parent_page"] = forms.ModelChoiceField(
    initial=self.page.get_parent(),
    queryset=Page.objects.all(),
    widget=widgets.AdminPageChooser(target_models=self.page.specific_class.allowed_parent_page_models(), can_choose_root=True, user_perms="copy_to"),
    label=_("New parent page"),
    help_text=_("This copy will be a child of this given parent page."),
)

Following the example on bakerydemo and taking as reference the mentioned change, the page selector would look like this

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