[web] Add "Save Avatar" button to EmojiAvatarSelectionModal

Summary:
For now just sets user avatar to pendingEmojiAvatar.

In subsequent diffs we will handle success/failure states properly.

Depends on D8269

Test Plan:
Works as expected:

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

Reviewed By: rohan

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8278