[web] Misc renaming in MessageActionButtons component

Summary: Changed some previously implementation-specific variable names to be more generic

Test Plan: Find and replace

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

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3312