FEP-1b12: Group federation

[silverpill]

@SorteKanin

This doesn’t sound particular scalable, if we imagine more group implementations coming - and what if I want to support both FEP-1b12 groups and another group implementation at the same time? I’d need to decide on the group types according to some ad-hoc rules of which groups have which properties? Again, I really think an explicit signal is preferable here.

I have found that the most reliable indicator of a FEP-1b12 Group is attributedTo (or equivalent). Unlike boost-bots, FEP-1b12 groups are moderated.