[web] Return dispatchCreateSubchannel directly in Stepper.Item nextProps

Summary:
While working on the unused-promise Flow lint, I noticed that we can simplify this code if we adjust the API in the Stepper component to allow mixed instead of void returns.

Depends on D10162

Test Plan: Flow

Reviewers: tomek

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D10163