Add Download Options for Gutenberg Books

[j-schwartz]

A user reports back they would like to read Gutenberg books on tablets offline / disconnected from RACHEL, but unlike African Storybooks, Gutenberg books cannot get downloaded to a tablet

[needlestack]

African Storybooks is PDF, and as it stands Gutenberg only releases HTML and epub - and then we only include their HTML. I would have to check whether epub downloads to tablets - if so we could update the Gutenberg module to include those. Or switch to the kiwix-based Gutenberg module which isn't as pretty but includes epub...