Fix circular import for PageInterface when using custom page interface

[mgax]

I'd like to define a custom PageInterface (using the PAGE_INTERFACE setting). Because grapple.types.pages.PageInterface is quite complex, I'd like to inherit from it, and add some additional fields.

This results in a circular import, because the definition of the Page class in the same module calls get_page_interface:

wagtail-grapple/grapple/types/pages.py

Lines 165 to 173 in d9cd8f2

	class Page(DjangoObjectType):
	"""
	Base Page type used if one isn't generated for the current model.
	All other node types extend this.
	"""
	class Meta:
	model = WagtailPage
	interfaces = (get_page_interface(),)

---

This PR moves PageInterface and get_page_interface to a new module, interfaces.py.

[mgax]

CI is failing for the coverage job, but it looks like all recent CI runs are failing on that.

[zerolab]

the actions/upload-artifact step needs include-hidden-files: true (example, reference)

[zerolab]

Since we're making these changes, it may be worth moving the StreamFieldInterface in the new interfaces module

[mgax]

Since we're making these changes, it may be worth moving the StreamFieldInterface in the new interfaces module

Ah, I had meant to do that, but got carried away. Thanks!

[zerolab]

note to self: follow up with clean-up PR

[zerolab]

LGTM