[lib] introduce getAvatarForUser in avatar-utils

Summary:
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 this emoji 👤 for the user.

This diff won't be landed until there is a feature flag for the avatars work

Linear Task: https://linear.app/comm/issue/ENG-3257/create-function-to-automatically-select-an-emoji-avatar-for-users-with

Test Plan:
I rendered the avatar (coming later in the stack) and checked to make sure the default emoji avatars didn't change for the same user

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Reviewers: ashoat, atul

Reviewed By: ashoat

Subscribers: varun, ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7053