[web] Create a function that reassigns notifs session

Summary:
This function determines where a session were stored, where it should be placed, and moves it from one to another.

https://linear.app/comm/issue/ENG-7688/introduce-a-function-that-moves-a-notifs-session

Depends on D11647

Test Plan: Tested in combination with the next diff: open the web app, login, send a message from another client - a notif should be displayed (without this stack we can only see a notif with a warning message). Also, verify that the reassignment actually occurs by adding some console logs.

Reviewers: kamil, marcin, varun

Reviewed By: marcin

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D11648