[web] Align MessageTimestampTooltip under MessageTooltip

Summary:
Part of a stack of diffs to fix the timestamp bug, related Linear issue here.

This diff moves the timestamp tooltip below the message tooltip (with the reply and create thread buttons). Handling other front-end changes like color will be handled in subsequent diffs in this stack, this diff is just focused on aligning the tooltip correctly.

Depends on D4481.

Test Plan:
Tested on Chrome/Safari and looks as expected.
Chrome:

Safari:

Additional stills:

Reviewers: atul, jacek, tomek

Reviewed By: atul

Subscribers: ashoat, adrian

Differential Revision: https://phab.comm.dev/D4704