improve handling of hidden peers

[Alexander Cobleigh]

• keep the first three letters of the nick, erase the rest and replace with e.g. the first 6 characters of their public key
• stop propagating/acting on user update events in cabal-client from peers that are hidden if (user.isHidden()) return

so fuckedUpTroll becomes fuc-f8a1b4. this still allows identification in case something went wrong, or people are just trying out moderation actions for the first time (don't wanna hide nickwarner and then be unable to reverse the hide!!)

the moderation messages that are forwarded to others also need to maintain the nickname such that people can know who has been hidden by whom, otherwise the moderation system becomes inscrutable

extracted from #192 (comment)