User can only select one avatar

[IDragonfire]

Currently every AvatarAssignment (table avatars) has a property selected.
Potentially a user can select multiple avatars.

Avatar selection is currently handled by the server ...
https://github.com/FAForever/server/blob/f5c056263fddde5edc4c7927abed61d0fd91e718/server/lobbyconnection.py#L853

1. Avatar Handling should be moved to the API
2. The database model should be adapted, e.g. as an column of login or 1:1 table avatars:login