[lib] Reducer for commServicesAccessToken

Summary:
Modelled this on lib/reducers/data-loaded-reducer.js.

Depends on D7241. Linear task: ENG-3389.

Test Plan: Flow

Reviewers: kamil, varun, jon, atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7269