[web] Move reducing logic for picking a community to its own reducer

Summary: Addressing review on D6969 - creating a separate reducer for picked communities

Test Plan: Run web app, checked that pressing a drawer item in the Calendar tab still changes the state properly

Reviewers: kamil, tomek

Reviewed By: tomek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D7341