As a Wikisource user, I want the WSExport gadget to be migrated to the Wikisource extension, so that I can access links to download books (via WSExport) on all wikis with the extension enabled.
**Background:** In the process of consulting with Wikisource users on the [[ https://meta.wikimedia.org/wiki/Talk:Community_Tech/Ebook_Export_Improvement#Have_we_covered_the_main_methods_to_export_ebooks? | project talk page ]], we have learned that some users don't have the ability to click download links (such as "Download as MOBI") via the side panel and other places. This is the case on Czech Wikisource, for example. This is because they do not have the WSExport gadget enabled on their wiki. Rather than enabling it on each wiki, it would be more efficient to migrate the gadget over to the Wikisource extension. That way, all wikis that have the extension enabled can have access to the gadget. One important note about this ticket is that it overlaps with another wish ([[ https://meta.wikimedia.org/wiki/Community_Wishlist_Survey_2020/Wikisource/Migrate_Wikisource_specific_edit_tools_from_gadgets_to_Wikisource_extension | Migrate Wikisource specific edit tools from gadgets to Wikisource extension ]]). However, it may make sense to include this as part of the ebook export improvement project, as many users cannot easily download books in their wikis at this time.
**Acceptance Criteria:**
* Migrate the [[ https://en.wikisource.org/wiki/MediaWiki:Gadget-WSexport.js | WSExport gadget ]] to the [[ https://www.mediawiki.org/wiki/Extension:Wikisource#:~:text=Wikisource%20is%20an%20extension%20to,It%20depends%20on%20WikibaseClient. | Wikisource extension ]]
**Pages to be updated**
[x] https://eo.wikisource.org/wiki/MediaWiki:Common.js
[x] https://fr.wikisource.org/wiki/MediaWiki:Common.js
[x] https://fr.wikisource.org/wiki/MediaWiki:Gadget-stockText.js (no action needed)
[x] https://fr.wikisource.org/wiki/MediaWiki:Gadget-WSexport.js (deleted)
[x] https://fr.wikisource.org/wiki/MediaWiki:Mobile.js (no action needed)
[x] https://de.wikisource.org/wiki/MediaWiki:Common.js
[x] https://tr.wikisource.org/wiki/MediaWiki:Gadget-WSexport.js (removed from Gadgets-definition, now can be deleted)
[x] https://is.wikisource.org/wiki/Melding:Common.js
[x] https://la.wikisource.org/wiki/MediaWiki:Common.js
[x] https://it.wikisource.org/wiki/MediaWiki:Gadget-ePubDownloadLink.js (not used in Gadgets-definition, can be deleted)
[x] https://pl.wikisource.org/wiki/MediaWiki:Common.js (this wiki had links for extra formats, see [[ https://pl.wikisource.org/wiki/Special:Diff/2686714 | Special:Diff/2686714 ]])
[] https://pl.wikisource.org/wiki/MediaWiki:Mobile.js (leaving alone for now since extension does not add download links to mobile)
[] https://es.wikisource.org/wiki/MediaWiki:Common.js
[] https://en.wikisource.org/wiki/MediaWiki:BookMaker.js
[] https://en.wikisource.org/wiki/MediaWiki:Mobile.js
[] https://en.wikisource.org/wiki/MediaWiki:Gadget-WSexport.js
[] https://gu.wikisource.org/wiki/મીડિયાવિકિ:Gadget-WSexport.js
[] https://pt.wikisource.org/wiki/MediaWiki:Gadget-WSexport.js
[] https://ta.wikisource.org/wiki/மீடியாவிக்கி:Mobile.js
[] https://ta.wikisource.org/wiki/மீடியாவிக்கி:Gadget-stockText.js
[] https://ta.wikisource.org/wiki/மீடியாவிக்கி:Gadget-WSexport.js
[] https://ca.wikisource.org/wiki/MediaWiki:Common.js
[] https://ca.wikisource.org/wiki/MediaWiki:Mobile.js
[] https://nl.wikisource.org/wiki/MediaWiki:Gadget-WSexport.js
[] https://ru.wikisource.org/wiki/MediaWiki:Common.js
[] https://zh.wikisource.org/wiki/MediaWiki:Gadget-WSexport.js
[] https://eu.wikisource.org/wiki/MediaWiki:Common.js
[] https://hy.wikisource.org/wiki/MediaWiki:Gadget-WSexport.js
[] https://uk.wikisource.org/wiki/MediaWiki:Gadget-stockText.js
[] https://uk.wikisource.org/wiki/MediaWiki:Gadget-WSexport.js
[] https://sv.wikisource.org/wiki/MediaWiki:Common.js
[] https://da.wikisource.org/wiki/MediaWiki:Common.js
[] https://da.wikisource.org/wiki/MediaWiki:Mobile.js
[] https://et.wikisource.org/wiki/MediaWiki:Common.js
[] https://fa.wikisource.org/wiki/مدیاویکی:Common.js
[] https://vi.wikisource.org/wiki/MediaWiki:Gadget-stockText.js
[] https://vi.wikisource.org/wiki/MediaWiki:Mobile.js
[] https://vi.wikisource.org/wiki/MediaWiki:Gadget-WSexport.js
[] https://th.wikisource.org/wiki/มีเดียวิกิ:Mobile.js
[] https://th.wikisource.org/wiki/มีเดียวิกิ:Gadget-WSexport.js
[] https://bn.wikisource.org/wiki/মিডিয়াউইকি:Gadget-WSexport.js
[] https://bn.wikisource.org/wiki/মিডিয়াউইকি:Gadget-stockText.js
[] https://bn.wikisource.org/wiki/মিডিয়াউইকি:Mobile.js
[] https://sa.wikisource.org/wiki/मीडियाविकि:Gadget-WSexport.js
[] https://vec.wikisource.org/wiki/MediaWiki:Gadget-ePubDownloadLink.js
[] https://kn.wikisource.org/wiki/ಮೀಡಿಯವಿಕಿ:Gadget-WSexport.js
[x] https://or.wikisource.org/wiki/ମିଡ଼ିଆଉଇକି:Common.js
[x] https://el.wikisource.org/wiki/MediaWiki:Common.js
[] https://pa.wikisource.org/wiki/ਮੀਡੀਆਵਿਕੀ:Gadget-WSexport.js
[x] https://wikisource.org/wiki/MediaWiki:Common.js