[web] Add box shadow to message tooltips

Summary: Added box shadow to message timestamp tooltip and message tooltip components. The changes are subtle but they definitely make the tooltips "pop" more

Test Plan:
Please view the screenshots to see the before and after of the changes I made:

Before:

After:

Reviewers: atul, abosh

Reviewed By: atul, abosh

Subscribers: ashoat, tomek, atul, abosh

Differential Revision: https://phab.comm.dev/D5111