[web] split the inline engagement pill into two

Summary:
As a first step to redesigning the InlineEngagement component we should split the sidebar and the reactions into two separate pills.

This diff and subsequent diffs won't be landed until the rest of the web inline engagement work is completed

Depends on D8677

Test Plan:
Please look at the screenshots below:

Before:

After:

Reviewers: atul, kamil

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8678