Extract load_image() function

This code was basically duplicated between load_item_and_property() and
load_file(), for no good reason as far as I can tell. load_file() needs
the page ID (unlike load_item_and_property()), but it doesn’t cost us
anything to query that (it’s always included in the response, it doesn’t
have a separate prop that you could leave out to save bandwidth or
anything), so we can just always return that as the image_page_id, and
then have load_file() translate that into the entity_id.

This prepares for adding the image size (width, height) to the HTML,
which is another piece of API information that shouldn’t be duplicated.