[Task] Add page number to information in document_sources field on Map

[mradamcox]

Describe the task

The document_sources property on Map objects is essentially just a list of urls pointing to the source image of the document. Currently, as a leftover from the LOC parser, that url is interrogated when the Document objects are created, and a page number is inferred from them, which is then attached to the Document object. It would be better to determine the page number at the time that the document_sources list is created. This would help the underlying issue with slugs that caused #213.

The structure of the document sources list should/could be like this:

[
  {"url": "url for the document", "page_number": "1"},
  {"url": "url for the document", "page_number": "2"}
]

Note that page_number can be non-numbers, as they will ultimately be naturally sorted.

Implementing this will presumably require moving the generation of a document_sources list upstream into the importer's parsing method, which will then pass the list directly into the newly created Map instance.