[web] introduce clear all button 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 clear all button + clear all functionality for the add users list

Linear task: https://linear.app/comm/issue/ENG-5958/clear-all-selected-functionality

Depends on D10745

Test Plan:
Please see the demo video below

Reviewers: atul, inka

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10746