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