Provide a UI for accessing content of inaccessible ZIMs

[Jaifroid]

As noted in openzim/gutenberg#75 , some ZIMs, such as recent Gutenberg, PhET and TEDx ZIMs, have a non-standard UI to access their content. In each case, the UI relies on the client being able to execute the JavaScript in the landing page, and this JavaScript is often inline, something that many Content Security Policies forbid (e.g. in extensions and apps).

We therefore need to provide an alternative means to access the content of these ZIMs, other than simple title search, as we often cannot easily know in advance what to look for (we would have to know the exact title of a TED talk to be able to find it, for example).

While it might be possible to emulate each ZIM's proprietary UI, and extract the same data, it would have to be done for each ZIM type and kept up-to-date. Therefore we need a more basic method to make such ZIMs accessible.

I propose a generic, scrollable title listing, with traditional alphabetical selector. It makes all these ZIMs much more usable. I provide screenshots below. I'd be happy to backport it, or something similar, upstream to Kiwix JS if it would be useful.

[Jaifroid]

This uses a Bootstrap modal to display the Index, but it can also be displayed in the main body of the app (the article div) perfectly well. The ZIM here is the latest dirtybiology.

[Jaifroid]

The main function for this can be seen here.

[Jaifroid]

The Archive Index UI now supports user-selectable Unicode "alphabets" (a field on the Settings page allows user to select first and last characters of alphabet string):

[Jaifroid]

This has now been merged to master.