Introduce `getAvatarForUser` in `avatar-utils`. `getAvatarForUser` is a function that checks if a user has an avatar, and either returns that or randomly generates an emoji avatar for the user and returns that. We generate an emoji avatar based on the user's `username`. If the user does not have a `username` set, we just default to an avatar with a smile emoji 😀 for the user.
**This diff won't be landed until the avatars stack is all accepted**
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Linear Task: https://linear.app/comm/issue/ENG-3257/create-function-to-automatically-select-an-emoji-avatar-for-users-with