Move SnippetViewSet.get_edit_handler() to ModelViewSet

[laymonage]

Part of #10740. This allows using the Panels mechanism to render the form for ModelViewSet by defining panels or edit_handler on the ModelViewSet or model class.

One difference from SnippetViewSet is that ModelViewSet does not fall back to extracting the panel definitions from the model class if neither panels nor edit_handler is defined, and instead will fall back to the original behaviour of requiring either form_fields, exclude_form_fields, or overriding get_form_class.

To test, you can use my bakerydemo branch where the Country model has been configured to use a ModelViewSet with panels.

When reviewing, hide whitespace to get better diffs.

Please check the following:

• Do the tests still pass?¹
• Does the code comply with the style guide?
  • Run make lint from the Wagtail root.
• For Python changes: Have you added tests to cover the new/fixed behaviour?
• For new features: Has the documentation been updated accordingly?

Footnotes

1. Development Testing ↩

[squash-labs]

Manage this branch in Squash

Test this branch here: https://laymonagemodelviewset-edit-han-je9ic.squash.io

[gasman]

LGTM!