The EmojiAvatarSelectionModal component was user avatar specific. In this diff we "lift" user avatar-specific logic to UserEmojiAvatarSelectionModal and make EmojiAvatarSelectionModal user/thread-avatar agnostic.
The "API" for EmojiAvatarSelectionModal will be the currentAvatar, defaultAvatar, setEmojiAvatar, and avatarSaveInProgress props. This should allow maximal reuse of EmojiAvatarSelectionModal component.
Depends on D8854