[web] Properly style messageActionButtonsContainer

Summary:
Before: https://blob.sh/atul/b611.png
After: https://blob.sh/atul/1349.png

Test Plan: NA

Reviewers: palys-swm, def-au1t, benschac, varun

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3307