[web] Add a selector for getting the item currently selected in the drawer

Summary:
issue: https://linear.app/comm/issue/ENG-2740/add-navigation-state-info-to-the-top-bar
Adding a selector for getting the item currently selected in the drawer, taking into account that in different tabs this value is stored differently

Test Plan: Log to the console the value returned when being in different tabs, checked that it is correct.

Reviewers: tomek, michal, kamil

Reviewed By: tomek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D6746