Switch from semi-official package to official APT repository
In my humble opinion we can then also release Briar to Flathub.org (#37).
To complete this issue, we should
- change the text in the readme
- push an last update to the semi-public sources, telling users to switch to the new version
I think starting with version %GTK 0.2.0-beta1 we should only offer updates through official sources.