setUserAvatar(userId: string,data: Uint8Array): Promise<User>
Makes the image data user userId's avatar and returns them, for an
upload (PUT /api/me/avatar) and for the avatar a user from another
instance has there (federation-service.ts). Presence alone would update
only the online list, so it also sends user_updated, which reaches every
avatar on screen, its bumped updatedAt busting their cache.
data: Uint8Array
Promise<User>