Use smaller license identifiers at files' top

data/app.briar.gtk.appdata.xml.in

src/briar-gtk.in

src/briar/api/api.py

src/briar/api/briar.py

src/briar/gtk/application.py

src/briar/gtk/container.py

src/briar/gtk/define.py

src/briar/gtk/logger.py

src/briar/gtk/window.py

src/meson_post_install.py

tools/generate_data.sh