Implement task/publish to create app website and publish/update flatpak repository to it

[aral]

Given that Flatpaks can be distributed in a decentralised manner, in the spirit of the Small Web, we should make this as easy to do as possible.

Proposed implementation

task/publish

Running this script:

1. Creates (or updates) a web site for the app based on the app metadata in the project.
2. Create (or updates) a flatpak repository for the app and a flatpak.ref and links it to the “Install” button on the site.

The site can then be served with Site.js.

Notes:

• Site is localised.
• People who install it also get updates.