Refactor Justin intro modal to separate it from avatar edit

[SabreCat]

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.

[adamfitzgibbon]

I'll start work on this one.

[Alys]

@adamfitzgibbon thanks! I've marked it as in progress for you.

[adamfitzgibbon]

As an update, I'm still planning on completing this issue, but as I'm new to Vue I'm going to take some low-hanging fruit to get a little more experience before finishing it.

[adamfitzgibbon]

I've gotten a little busier so will try to get around to doing this, but if anyone else is looking for something to try, feel free to let me know and grab it from me.

[Alys]

@adamfitzgibbon Thanks for letting us know! I've put this back to help wanted but post here if you'd like to take it up again.