Library issues

[EJMFarrow]

• Insert embedded files in rendered question text on STACK library page.
• Files are stored in plots folder. Will be a relatively minimal number but we might want to have a separate folder at some point. (Library was storing them here previously but not displaying them!) Library cache now purged when STACK cache purged.
• Handle markdown and other formats in library render.
• Fix deprecation error thrown by API test.
• Non-STACK questions no longer have a spurious link when imported as part of a quiz.
• Question text field on library page now shows actual text not HTML.
• Fix display of embedded files on question dashboard.

[sangwinc]

Thanks @EJMFarrow!