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/usr/lib/systemd/system-preset/70-kiosk.preset

@@ -0,0 +1,3 @@
+enable wpa_supplicant@.service wlan0
+
+enable kiosk.service