Require that custom page and snippet interfaces inherit from the defaults

When users of the library use a custom page interface by configuring PAGE_INTERFACE, or a custom snippet interface (which is being added in #405), the interface they provide should inherit from the default interface, so that it provides all the right fields. #405 (comment)

Add checks in get_page_interface and get_snippet_interface and raise a warning if the interface doesn't inherit from the default interface.