[web] Introduce ThreadEmojiAvatarSelectionModal

Summary:
Enables setting of emoji avatars on web. As of this diff all varieties of thread avatar can be set from web. We just need to make a few layout tweaks in ThreadSettingsModalGeneralTab in order to get this ready to ship.

Depends on D8855

Test Plan: Works as expected:

Reviewers: ashoat, ginsu

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8856