Proper welcome bot

[turt2live]

Not the silly welcome back bot, but a bot that can send a configurable message to the room when someone new joins.

[MilkManzJourDaddy]

@arathorn ¦ @matthew:matrix.org has commented that ACL «whisper» messages visible only to specific MXIDs in Public Rooms should probably be a thing.  We know some IRC net's gave similar for join¦greeting messages.  If such comes to Matrix, such a Greeting 'Bot, or (Un)Mute 'Bot #115 should «whisper» it's messsage to joiners instead of uncollapsing the join/part messages and adding room flooding traffic.  ↵↵¶That clients now collapse userlist change messages, especially on mobile, is greatly appreciated.  Undoing that would probably be greatly loathed despite the utility of such a 'bot.

[ptman]

hemppa has some welcome functionality