Create official APT repository

Since packaging briar-headless in Debian main (#1) is currently blocked, we decided to now focus on creating an official APT repository by The Briar Project. It will contain a .deb of briar-headless, produced by briar-reproducer (briar-reproducer#1 (closed)), and imho should also contain .debs of briar-gtk (briar-gtk#39 (closed)) and python3-briar-wrapper (python-briar-wrapper#14 (closed)).