[web] Move replyButton from ComposedMessage to MessageActionButtons

Summary: Having the replyButton in MessageActionButtons instead of ComposedMessage sets things up for later diffs...

Test Plan:
Things continue to work as they did before.
Two known issues that will be addressed in subsequent diffs (wanted to limit the scope of this diff)

• We want the icons to appear next to each other (flex-direction:row) instead of "on top" of each other
• We want to position the action buttons (sidebar and reply) conditionally based on isViewer

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

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3306