Add the ability to customize the UserViewSet via the app config

[laymonage]

Fixes #11954. Somewhat "solves" the step 2 in #11915 indirectly, albeit not yet documented (via IndexView.get_list_buttons/IndexView.get_list_more_buttons).

The mechanism follows what we already have for the GroupViewSet. We could go a step further by also deprecating the WAGTAIL_USER_EDIT_FORM, WAGTAIL_USER_CREATION_FORM, and WAGTAIL_USER_CUSTOM_FIELDS settings, but I thought I'd test the waters with a smaller-scoped PR for now.

[squash-labs]

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Test this branch here: https://laymonagecustom-user-viewset-w91r5.squash.io

[laymonage]

The currently documented way to customise the user forms (added in #2414) is a bit iffy. For one, I don't understand why we suggest specifying the custom fields in WAGTAIL_USER_CUSTOM_FIELDS, if the fields are already added declaratively in the custom forms.

With the example in https://docs.wagtail.org/en/stable/advanced_topics/customisation/custom_user_models.html, adding WAGTAIL_USER_CUSTOM_FIELDS = ['country', 'status'] really has no effect, as the fields are already specified declaratively on the form class. The setting is used here,

wagtail/wagtail/users/forms.py

Lines 27 to 33 in a09bba6

	# The standard fields each user model is expected to have, as a minimum.
	standard_fields = {"email", "first_name", "last_name", "is_superuser", "groups"}
	# Custom fields
	if hasattr(settings, "WAGTAIL_USER_CUSTOM_FIELDS"):
	custom_fields = set(settings.WAGTAIL_USER_CUSTOM_FIELDS)
	else:
	custom_fields = set()

which is then combined with the default fields in the form's Meta.fields:

wagtail/wagtail/users/forms.py

Lines 199 to 203 in a09bba6

	class UserCreationForm(UserForm):
	class Meta:
	model = User
	fields = {User.USERNAME_FIELD} | standard_fields | custom_fields
	widgets = {"groups": forms.CheckboxSelectMultiple}

However, as per Django docs,

ModelForm is a regular Form which can automatically generate certain fields. The fields that are automatically generated depend on the content of the Meta class and on which fields have already been defined declaratively. Basically, ModelForm will only generate fields that are missing from the form, or in other words, fields that weren’t defined declaratively.

If there are fields not specified declaratively on the form class, then yes, that setting could be useful. But that's not how the docs explain it...

I'm inclined to deprecate the settings as soon as we have this UserViewSet customisation publicly documented, for reasons mentioned in #6477 (comment), but I also know that the documentation could do a bit of rework, so I left it out for now. If you think that's a good idea, I'd be happy to add the deprecations and make a draft for the docs, though. Perhaps merging it with the GroupViewSet documentation https://docs.wagtail.org/en/stable/extending/customizing_group_views.html would also be a good idea...