[web] Introduce Stepper - common multiple step component

Summary:
Depends on D5185
Context: here

Introduce common Stepper component, for creating multiple step forms and (in the future) multiple page modals.

The component receives steps, information about active step and steps setter from parent - it doesn't keep the state with current active step itself.

For better code cleanliness, to set activeStep, we use children key property instead of using array indexes.

Test Plan: This component will be used in next diff (firstly in ComposeSubchannelModal). Currently, no changes in the app.

Reviewers: tomek, atul

Reviewed By: tomek

Subscribers: ashoat, tomek, atul, abosh

Differential Revision: https://phab.comm.dev/D5186