[web] Remove extraneous sidebarExistsOrCanBeCreated prop in MessageActionButtons

Summary: Can use useSidebarExistsOrCanBeCreated hook directly instead of "drilling" it in.

Test Plan: Close reading, things seem to continue working as expected

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

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3310