Flair for bridged users federation-wide

This is heavily dependent on a way to identify bridged users in the federation (https://github.com/matrix-org/matrix-doc/issues/646). Somehow this should also be supported in a decentralized way so other bridge providers (ie: not matrix.org) can also get their flair out there, however there is concern for abuse of the system here.

The idea is that Discord, IRC, etc users all get flair for their relevant groups (communities) regardless of where they speak. Currently there are ways to simulate this during the provisioning process (portal rooms can just add the related groups state event; plumbed rooms can try and add the state event, but may fail).