[web] introduce previouslySelectedUsers map to add users list

Summary:
PLEASE NOTE THAT THIS DIFF AND SUBSEQUENT DIFFS IN THIS STACK WILL NOT BE LANDED UNTIL MORE OF THE REDESIGN IS READY SINCE THIS WILL CAUSE REGRESSIONS IN PROD

This diff introduces the previouslySelectedUsers state. Based on the design requirements in the figma we only want to move users in and out of the previously section whenever the modal view changes into "search mode" (whenever searchModeActive changes).

We should not jump the members on their own, they will only be moved when the modal “changes” views when a user searches and then clears.

Subsequent diffs will handle introducing the previously selected users to the UI.

Linear task: https://linear.app/comm/issue/ENG-5960/selected-users-section

Depends on D10819

Test Plan:
Please see the demo video below

Reviewers: atul, inka

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10820