Refactor image chooser pagination to check WAGTAILIMAGES_CHOOSER_PAGE_SIZE at runtime

[gasman]

Reworking of #11884, to fix #11813.

The problem with setting per_page as an attribute on the viewset is that it requires reading the setting on module load, which means we can't use override_settings to test it. Instead, make it a property on BaseImageChooseView so that it gets read back at runtime. (Which in turn means that the construct_view logic in the base ViewSet class needs to take care not to write to attributes on the view if they're actually properties.)

We could also have done this by adding a get_per_page method on the generic chooser view (defaulting to just returning self.per_page) and making it overrideable on the viewset using inject_view_methods - but we've pretty much settled on using properties rather than get_foo methods in our generic views now.

I also took the liberty of increasing the default page size to 20, since this bug inadvertently created an ambiguity over whether the "correct" value is 10 (which works for rows of 5) or 12 (which works for rows of 4) - this way it works equally well for both :-)

[squash-labs]

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Test this branch here: https://gasmanrohitsrma-fix-chooser-pa-9pr2k.squash.io

[laymonage]

I'm not sure if special-casing property is a good idea here... Sometimes we use cached_property on the view, and if we exclude property here, should we do the same for cached_property too?

I'm leaning towards defining a sentinel value for "undefined" configuration in the viewset as you described in #11162 (comment), and excluding that sentinel here, instead of specifically checking for property/cached_property. It would allow us to explicitly say "use whatever the view class has" for any attribute, instead of doing something like the following:

wagtail/wagtail/admin/viewsets/model.py

Lines 469 to 500 in 538365f

	@cached_property
	def search_fields(self):
	"""
	The fields to use for the search in the index view.
	If set to ``None`` and :attr:`search_backend_name` is set to use a Wagtail search backend,
	the ``search_fields`` attribute of the model will be used instead.
	"""
	return self.index_view_class.search_fields
	@cached_property
	def search_backend_name(self):
	"""
	The name of the Wagtail search backend to use for the search in the index view.
	If set to a falsy value, the search will fall back to use Django's QuerySet API.
	"""
	return self.index_view_class.search_backend_name
	@cached_property
	def list_export(self):
	"""
	A list or tuple, where each item is the name of a field, an attribute,
	or a single-argument callable on the model to be exported.
	"""
	return self.index_view_class.list_export
	@cached_property
	def export_headings(self):
	"""
	A dictionary of export column heading overrides in the format
	``{field_name: heading}``.
	"""
	return self.index_view_class.export_headings

which fails if the view defines those attributes as property/cached_property, as you'd be passing the unbound version of the view's property to the view instance during as_view(). For example, some views override columns as a cached_property to allow accessing the self view instance to call its methods.

[gasman]

Superseded by #11937