[web] Conditionally lay out MessageActionButtons based on isViewer

Summary: Match existing layout (reply button closer to message)... fixes issue knowingly "introduced" in D3306

Test Plan:
!isViewer: https://blob.sh/atul/400a.png
isViewer: https://blob.sh/atul/7c56.png

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

Reviewed By: palys-swm, ashoat

Subscribers: ashoat, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3309