Better and especially across all platforms working way for handling /myroomnick and /myroomavatar

[su-ex]

/myroomnick is a bit broken. On riot-web it's a bit working (nick for the current room is changed) but the global nick also appears changed in settings although for all the other rooms it definitely stays unchanged.
On riot-android /myroomnick to set the room nick is not available at all, but the behaviour is the same like described above.

Also it's very weird to set avatars via those mxc_urls (how to get those at all?).

A really nice way would be to have some sort of profiles and the possibility to automatically attach rooms to it based on a ruleset when to apply which one (default public, same server, direct chats with one person, ...) and to manually attach them to rooms to be able to change the nick and avatar for a whole group of rooms all at once.

Currently the handling of nick and avatar might be a privacy issue because they are publically visible without an option to show it only to members of rooms you're in with or so.