[web] Introduce placeholder EmojiAvatarSelectionModal

Summary:
Introduce placeholder EmojiAvatarSelectionModal component. Modal opens when "Select emoji" MenuItem in EditUserAvatarMenu is clicked.

Layout/styling/functionality will be introduced incrementally in subsequent diffs.

Depends on D8264

Test Plan:
Opens and looks about as expected:

Reviewers: ashoat, ginsu, kamil, michal, inka, rohan

Reviewed By: rohan

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8265