[web] Rename MessageActionButton to MessageActionButtons

Summary: After subsequent diffs, this component will be the parent of the sidebar and reply buttons.

Test Plan: NA, just separating this out for easier review

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

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3304