Feature request: A standardised API to allow developers to customise model edit forms based on the current user

Issue Summary

In Django ModelAdmin there’s a get_field_queryset(self, db, db_field, request) method, which allows the filtering of items displayed in the aforementioned fields based on the request.

https://github.com/django/django/blob/3f8ee58ccc5c55e62625ad797ddde05778fe1bec/django/contrib/admin/options.py#L206-L217

In the Wagtail ModelAdmin such method does not exist.

Is there a way to achieve a similar functionality as with Django get_field_queryset()?

[ababic]

In addition to the above, I think it might also be nice to bring in some additional flexibility to the get_edit_handler() method on Page, by allowing it to accept optional object and request arguments (a bit more like the version on the ModelAdmin class:https://github.com/wagtail/wagtail/blob/master/wagtail/contrib/modeladmin/options.py#L399), and trying to be consistent about providing those values in the various editing UIs. This would make it easier to do things like following (all of which I have done at some point):

• using different edit handlers classes/panel sets for create and update views
• surfacing different fields to different levels of user
• modifying the edit interface based on an object-specific field values

[ababic]

@gasman I have renamed/reopened this as, following a discussion on Slack, I feel it makes sense to have a ticket here to discuss (if anything) what we can do to provide a better API for this.