build: Have separate 'cssjanus' target for demo.rtl.css
build: Have separate 'cssjanus' target for demo.rtl.css

It's not part of the distribution.

Change-Id: I8e5f9ba33dffe5051b47bc899847359fd50e0e1c

wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui