[web] Change messageActionButtons svg color on hover

Summary:
Here's what it looks like:

Test Plan: Looks as expected

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

Reviewed By: palys-swm, benschac

Subscribers: Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3357