Extract revision and draft state logic from generic views into mixins

[laymonage]

Fixes #9710.

An alternate approach to #9190. I initially tried to create a separate mixin for each corresponding model mixin, and dynamically applying the view mixins in SnippetViewSet based on the model class. However, that seems to add more complexity as we'll need to have one view mixin for each model, for each view (e.g. CreateViewDraftStateMixin, EditViewDraftStateMixin). It will be very hard to make sure all the combinations work correctly, especially with the upcoming LockableMixin (and soon WorkflowMixin) integrations.

After tinkering around, I think it's worth to just combine all the model mixins logic into a single mixin for each view (XViewOptionalFeaturesMixin). This allows us to handle the model mixin combinations in each method in a simpler way (using if checks), while still removing the model mixin logic from the generic views.

I'd suggest reviewing each individual commit. The last commit combines the mixins for CreateView and EditView into a single mixin as some methods are exactly the same (publish_action(), form_valid(), etc.). Not sure if the reusability vs readability tradeoff is worth it, though. I'd be happy to remove that commit if desired.

I also noticed some method overrides in snippets views are no longer necessary as of #9221, so I removed them.

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.

Please describe additional details for testing this change.

Footnotes

1. Development Testing ↩

[squash-labs]

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Test this branch here: https://laymonagerefactor-generic-view-kxfpy.squash.io

[gasman]

Thanks - I'm happy with all of this! Good to have these special cases separated out from the rest of the admin.views.generic.models logic.