Support the onboarding of users during an Internet blackout

[gnarea]

Is your feature request related to a problem?

Yes. Many potential users of Awala may not have heard about it until they find themselves in an Internet blackout, but they should still be able to start using it without the Internet.

Describe the solution you'd like

A Awala User's Pack: A collection of some/all of the following files, which will be distributed by couriers and other users:

• The signed installer for Windows. (https://github.com/relaycorp/relaynet-gateway-desktop)
• The signed APK for Android (https://github.com/relaycorp/relaynet-gateway-android; see https://issuetracker.google.com/issues/180927886)
• A PDF to explain to non-technical users what Awala is about and how to install it on Windows/Android. This PDF should be signed by Relaycorp so that people can be certain of its authenticity if they open it with Acrobat Reader. This document would be available in multiple languages.
• The Windows/Android installers for a general-purpose Awala app, like Letro.

The courier apps could run an HTTP server to share those files when users are disconnected from the Internet, similar to how the Knapsack desktop app does it -- In fact, the files above could be great candidates to broadcast over Knapsack. To keep things simple, at least initially, couriers should be able to disable this functionality to save disk space, but they shouldn't be able to customise the files.

The files above could also be distributed over BitTorrent/IPFS and sneakernets like El Paquete Semanal.

Additional context

See also #2