[web] Introduce Stepper - common multiple step component
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