feat req: nicknames

[nyxmeowmeow]

id like to be able to set a different name for certain people, as their signal name is different to what i know them as. it also feels a bit strange seeing my own name in the list, so id like to be able to also change that to something like <self>

[nyxmeowmeow]

apparently signal does actually have a nicknames feature, but gurk doesnt show them