Refactor Justin intro modal to separate it from avatar edit

Description

creatorIntro.vue is used both for initial avatar setup on account creation, and for editing a user's avatar via the user menu. This makes it difficult to consistently position both modals, because ideal positioning for the Justin sequence pushes the much larger avatar edit modal partially offscreen.

What we should do is pull the shared avatar-related pieces out to a component that's then plugged into two separate modals, Justin and post-setup avatar editing. Then, reposition the Justin sequence so it's centered better onscreen.