[web] Modify Button so that onClick function can return any type

Summary: Our current button version doesn't work with async functions - which return promises. Instead of using void we should use mixed because we don't care about the returned value.

Test Plan: Flow

Reviewers: def-au1t, atul, benschac

Reviewed By: atul

Subscribers: ashoat, Adrian, yayabosh

Differential Revision: https://phab.comm.dev/D4265