Add functionality for External Links on Ebook Listing Pages

[bdr99]

This PR implements the following changes

• Add a function called getExternalLinks in a new file called external.py
  • This function takes a Book object as an argument, and is responsible for making API calls to Librivox and Standard Ebooks and returning a dictionary of URLs to those sites' listings for the given book
  • All API responses are cached using the CacheControl library which has been added to the project dependencies in requirements.txt. This will prevent unnecessary API calls.
  • We also had to update to the latest version of the requests library because we were using a very old version which was not compatible with the CacheControl library.
  • Librivox API calls are made using the following logic:
  • Request all books with the matching author last name.
  • If any results have the exact matching Gutenberg ID in their url_text_source field, then return the URL to the Librivox listing for that result.
  • If there were no results with the Gutenberg ID, then check to see if any have the exact matching title.
  • If there are none, then there is no result.
  • Standard Ebooks does not have an API, but it does have an OPDS catalog which contains a list of all of their ebooks. The logic used for this is as follows:
  • Retrieve the entire XML document for the OPDS catalog
  • For each book entry, if it has a matching Gutenberg ID, return the URL to the Standard Ebooks listing for that book.
  • If there were no matching entries, then there is no result.
• Add a view called ExternalLinkView
  • This view uses the getExternalLinks function described above to provide an endpoint for AJAX calls to retrieve the external links for a given Book ID.
  • The external links are returned in JSON format.
• Add code to the Ebook Listing Pages to show the external links
  • The Ebook Listing Page performs an AJAX call to the ExternalLinkView to retrieve the links for the book.
  • When the request completes, the results are dynamically inserted into the "Other Ways to Read This Book" section of the page.

[eshellman]

In production.

Maybe the display for other books by author could use tweaking

https://www.gitenberg.org/book/76