add list of moderators to top of Tavern chat

Uses "moderatorList" in the a moderatorList.json file, and uses moderatorIntro1 and moderatorIntro2 (in groups.json) for associated text.

See https://github.com/HabitRPG/habitrpg-shared/pull/239 for the JSON data that this uses.

I've done this in the most basic, simple way possible since it's likely that this message won't be needed for long but I can pretty it up if you want me to.

Ref: https://plus.google.com/u/0/112627489415765168293/posts/ZRn4EMFU3HQ  - discusses upcoming changes that will make moderators more obvious without this text, and the last few comments are about making this change as a stop-gap measure.