Custom app should be able to download zim from external URL

[Popolechien]

Here is the scenario:

• We want to produce an app with an extremely large zim file (e.g. more than 2 Gb), so much so that going through the playstore is not a viable option;
• User would therefore download a "shell app" (ie Kiwix-custom") which would in turn download and install the target zim file after it's been installed;
• (maybe a separate ticket?): the app would check when there's a new zim available and flag it to the user one way or the other (for instance with an "update content" button in the settings that would be greyed out when no such new content is available; I'd be wary of automated updates as users often are in atypical connectivity configurations).

@macgills 1. Would that be possible? and 2. How many days of dev work do you reckon it would take to implement the "separate download" approach?

[macgills]

See my answer on #61 we already have a system like this in place. Updates are a different beast though.

The build system currently doesn't have anyway to know about this new type of output as all it does is detect a tag and then upload to the playstore after building the right app.

This new app would need the appropriate icon/json to be registered and we would need some manner of publishing. If we don't care about a cohesive build system I can build a custom app locally easily.