[web] Introduce UserEmojiAvatarSelectionModal component

Summary:
The EmojiAvatarSelectionModal component was user avatar specific. In this diff we "lift" user avatar-specific logic to UserEmojiAvatarSelectionModal and make EmojiAvatarSelectionModal user/thread-avatar agnostic.

The "API" for EmojiAvatarSelectionModal will be the currentAvatar, defaultAvatar, setEmojiAvatar, and avatarSaveInProgress props. This should allow maximal reuse of EmojiAvatarSelectionModal component.

Depends on D8854

Test Plan:
Setting user emoji avatar continues working as expected/before.

Reviewers: ashoat, ginsu

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8855