[web] Introduce and style messageActionContainer for ComposedMessage

Summary:
Before: https://blob.sh/atul/36ae.png
After: https://blob.sh/atul/5b10.png

Illustration of messageActionActiveArea: https://blob.sh/atul/aa2b.png

Test Plan: NA

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

Reviewed By: benschac, ashoat

Subscribers: ashoat, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3283