[web] Finish styling MessageTimestampTooltip

Summary:
Relevant Linear issue here.

This diff adds styling changes to MessageTimestampTooltip, specifically changing the color of the tooltip, increasing the width of the tooltip (so it is at minimum the width of the MessageTooltip with the reply/create thread buttons), some padding, and makes the border radius match the MessageTooltip.

Depends on D4704.

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

Safari:

Additional stills:

Finally, the timestamp does not appear behind photos, as @ashoat noted on Linear:

Reviewers: atul, jacek, tomek

Reviewed By: atul, jacek

Subscribers: ashoat, adrian

Differential Revision: https://phab.comm.dev/D4705