[web] Move MessageActionButtons-related styles inside component

Summary:
I mistakenly put some of the styles for the MessageActionButtons (reply, create sidebar, etc) in the ComposedMessage stylesheet instead of in the MessageActionButtons stylesheet. This meant that the MessageActionButtons component was appearing incorrectly for RobotextMessages.

This diff puts the MessageActionButtons-related styling at the right layer of abstraction and fixes how things look for robotext messages.

Before:

After:

Test Plan:

1. Hover over a ComposedMessage and ensure that the MessageActionButtons component tooltip appears as it did before/as it does in Figma.
2. Hover over a RobotextMessage and ensure that the MessageActionButtons component appears as it does for ComposedMessages

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

Reviewed By: ashoat

Subscribers: ashoat, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3510