Similar logic to D7402 but for thread avatars. In addition to creating the selector, I needed to create a small utility function: getAvatarForThread which factors out most of the logic in useAvatarForThread but now we can easily use this logic in the savedEmojiAvatarSelectorForThread selector

Depends on D7424

All thread avatars still render properly (shows that getAvatarForThread works as expected) , and the reset functionality for thread avatars behaves as expected: