Install Syncthing through third-party repo

See https://bugs.launchpad.net/ubuntu/+source/syncthing/+bug/1847018

roles/desktop-common/tasks/packages.yml

@@ -18,6 +18,7 @@
     apt_key:
       url: "{{ item }}"
     loop:
+      - "https://syncthing.net/release-key.txt" # Syncthing
       - "https://packages.riot.im/debian/riot-im-archive-keyring.gpg" # Element
       - "https://download.spotify.com/debian/pubkey.gpg" # Spotify 1
       - "https://download.spotify.com/debian/pubkey_0D811D58.gpg" # Spotify 2
@@ -27,6 +28,7 @@
     loop:
       # Debs
       - "deb http://repository.spotify.com stable non-free" # Spotify
+      - "deb https://apt.syncthing.net/ syncthing stable" # Syncthing
       - "deb https://download.mono-project.com/repo/ubuntu vs-bionic main" # Monodevelop
       - "deb https://packages.riot.im/debian/ default main" # Element
       # First-party PPAs