Ensure all JSTOR article types work in picker

While testing the JSTOR APIs at https://labs.jstor.org/api/anno/docs I compiled a set of test cases of different types of JSTOR article, for use with the JSTOR content picker.

Test cases: https://gist.github.com/robertknight/a2b045b717d6c7dda18879df91c16e90

We should verify that all of these test cases are now handled as expected in the JSTOR content picker:

1. Create a new assignment in the LMS app
2. Choose JSTOR as the content type
3. Enter the article IDs from the test cases in the gist above

**Expected results:**

- If the article ID is valid and has a PDF: A title and thumbnail should be displayed
- If the article ID is valid, but there is no PDF: The title should be displayed, a thumbnail _may_ be displayed, but if not, it should fail with a 4xx error. The content picker should inform the user that no PDF is available and prevent them from choosing the article
- If the article ID is invalid: The title and thumbnail requests should fail with 4xx errors, and a sensible message should be displayed to the user
- The various "Special cases" in that gist should be handled as described