Personas feature (to support plural systems and roleplay) (WIP)

Hi, main dev for Sable's Per Message Profile implementation here. First off: this looks very similar, and that is good. I would emphasise that you should not name then Personas if possible. I'm currently trying to get this language changed in Sable to a more neutral "Profiles" because some view this language as disrepectful. (Subprofiles is a fine name, even.) The picker should also ideally be reactive to when the user inputs a proxy tag. (IE changing when a match is found.)