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/pam.d/kiosk

@@ -0,0 +1,7 @@
+account required pam_localuser.so
+
+session optional pam_loginuid.so
+session required pam_env.so envfile=/etc/profile.env
+session required pam_limits.so
+session required pam_env.so
+session required pam_systemd.so