feat(debian): APT config files

.config/debian-apt/signal.sources

@@ -0,0 +1,9 @@
+X-Repolib-Name: signal.org
+Enabled:        yes
+Types:          deb
+URIs:	        https://updates.signal.org/desktop/apt
+Suites:         xenial
+Components:     main
+Architectures:  amd64
+Trusted:	no
+Signed-By: /usr/share/keyrings/signal-desktop-keyring.gpg

.config/debian-apt/unstable.sources

@@ -0,0 +1,9 @@
+X-Repolib-Name: Debian Unstable
+Enabled:        yes
+Types:          deb
+URIs:	        http://deb.debian.org/debian/
+Suites:         unstable
+Components:     main contrib non-free-firmware
+Signed-By:	/usr/share/keyrings/debian-archive-keyring.gpg
+
+# Note: Remove files from /etc/apt/trusted.gpg.d/