[web] introduce barebones list header section in 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

As part of the new add users list ui we have a header section with two parts: a text to show how many users were selected and a clear all button. This diff introduces the structure for this header section and the selection text, and a subsquent diff will handle the clear all button logic

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

Depends on D10743

Test Plan:
Please see the demo video below

Reviewers: atul, inka

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10745