Add #osm-cwg to irc.osm.org drop-down

Add "#osm-cwg" to the channel drop-down appearing on https://irc.openstreetmap.org .

This is the IRC channel we use for Communication Working Group meetings, and between-meeting coordination. There was some [discussion of this idea in CWG meetings](https://wiki.osmfoundation.org/wiki/CWG_meeting_2018-04-12). I think it originated from a desire to be more inviting, encouraging people to join our working group or just visit our meetings.