Support using avatars from username history
Support using avatars from username history

Now you can use `/avatar` for any private avatar you have access to
from your username history, without needing to change name every time.

xfix/Pokemon-Showdown,svivian/Pokemon-Showdown,Zarel/Pokemon-Showdown,svivian/Pokemon-Showdown,svivian/Pokemon-Showdown,sirDonovan/Pokemon-Showdown,Zarel/Pokemon-Showdown,xfix/Pokemon-Showdown,xfix/Pokemon-Showdown,Zarel/Pokemon-Showdown,sirDonovan/Pokemon-Showdown,svivian/Pokemon-Showdown,xfix/Pokemon-Showdown,svivian/Pokemon-Showdown,xfix/Pokemon-Showdown,sirDonovan/Pokemon-Showdown