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/kiosk.service

@@ -0,0 +1,31 @@
+[Unit]
+After=systemd-user-sessions.service plymouth-quit-wait.service
+Before=graphical.target
+ConditionPathExists=/dev/tty1
+Wants=dbus.socket systemd-logind.service
+After=dbus.socket systemd-logind.service
+Conflicts=getty@tty1.service
+After=getty@tty1.service
+Wants=time-sync.target
+After=time-sync.target
+
+[Service]
+StateDirectory=%N
+CacheDirectory=%N
+Environment=XDG_CACHE_HOME=%C/%N
+ExecStart=/usr/bin/sway -c /etc/sway/kiosk.conf
+User=kiosk
+StandardInput=tty
+StandardOutput=tty
+StandardError=journal
+TTYPath=/dev/tty1
+TTYReset=yes
+TTYVHangup=yes
+TTYVTDisallocate=yes
+PAMName=kiosk
+UtmpMode=user
+UtmpIdentifier=tty1
+
+[Install]
+WantedBy=graphical.target
+Alias=display-manager.service