[web] [fix] add cursor to button component

Summary: button component should have cursor. Unable to screenshot, but normal button behavior.

Test Plan: hand shows up on hover

Reviewers: atul, ashoat

Reviewed By: atul, ashoat

Subscribers: ashoat, palys-swm, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3137