Background: Currently on enWS, we have some custom JS that adds an EPUB link to mobile page (at top right). It would be more consistent to be able to invoke the new Download dialog from a button in that position. The enWS JS is a gadget: https://en.wikisource.org/wiki/MediaWiki:Gadget-MobileDownloadLink.js

Acceptance Criteria:

• Add the WS Export "download" button to mobile version of Wikisource
• The pop-up generated by the download button should operate the same as on desktop
• If any UX considerations come up based on the UX differences of the mobile experience, they can be handled in a separate ticket (this is to just do the basic work of getting the button up & the ability to download via button)