Link nametag to headbone with offset instead of position offset

## Description When an avatar performs a sit emote, the nametag positioning is incorrect. The nametag appears to be offset from where it should be relative to the avatar's head. ## Observed Behavior The issue was observed with the PixelPixie avatar when reproducing the sit emote. The nametag does not follow the avatar's head position correctly during the animation. ## Proposed Solution Instead of using position with offset for nametag placement, the nametag should be linked to the avatar's headbone with an offset. This would ensure the nametag follows the head's actual position during animations. ## Related Issues - #418 (nametags cast shadows - different issue but related to nametag rendering) --- Requested by rob via Slack