Add an option in the interface to enable or disable JavaScript in the ZIM

[Jaifroid]

As we move towards enabling JavaScript in the ZIM -- already in master for SW mode, and a stable branch in jQuery mode -- I think we need to provide options to disable or enable it in the UI. I would argue that it should probably be off by default, as it is resource intensive: it requires extraction of up to 6 compressed JavaScript assets in some modern ZIMs. On slower mobile devices the performance hit and the battery guzzling could be unacceptable for everyday encyclopaedia-type usage, when it offers no real improvement for most ZIMs other than being able to tap headings to open and close their sections.

Disabling JS should be linked to #433 at least in jQuery mode, and arguably also in SW mode given the need to ensure all sections are open when JS-in-the-ZIM is disabled.