Begin implementing kiosk browser

This commit introduces the _kiosk.service_ unit, which launches `sway`
to start a Wayland session, which in turn launches Firefox.  The
`policies.json` file configures Firefox in a sort of kiosk mode,
disabling most features and blocking all but the desginated sites.
Unfortunately, running `firefox --kiosk` doesn't actually work: Firefox
apparently runs, but doesn't draw anything on the screen.

Note that we have to launch Firefox by its "real" path, since
`/usr/bin/firefox` is a Bash script, and Bash is not installed.
Fortunately, the wrapper script doesn't do anything we really care
about, so bypassing it is fine.

overlay/etc/sway/kiosk.conf

@@ -0,0 +1,14 @@
+# vim: set ft=swayconfig :
+
+output DSI-1 resolution 720x1280 transform 90
+
+input * {
+    map_to_output DSI-1
+}
+
+exec gsettings set org.gnome.desktop.interface gtk-theme Adwaita-dark
+exec gsettings set org.gnome.desktop.interface color-scheme prefer-dark
+
+exec /usr/lib64/firefox/firefox
+
+for_window [title="Mozilla Firefox"] fullscreen