[web] clean up InlineEngagement component

Summary:
Small cleanup diff to better organize and make the InlineEngagement component easier to read/go through. The two changes to note are

1. Renamed threadInfo to sidebarThreadInfo: This is what we did in D8496
2. Renamed messageLabel to editedLabel: In native we call the edited label editedLabel, so I updated this name to keep everything consistent.

Test Plan: Confirmed that InlineEngagment still looked and behaved the same.

Reviewers: atul, kamil

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8677