Switch from semi-official package to official APT repository

Once briar-debian#3 is done, we should only offer using that repository and deprecate (or delete) the existing semi-public package at media.dorfbrunnen.eu/briar/debs.

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.