[web] Style multimediaUpload button in ChatInputBar on hover

Summary:
Looks as expected:

Test Plan: NA

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

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3365