[web] update add user list header based on if searching users

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

When the user has inputted something into the search bar of the add user list, the UI should update to show that the user is in the "search mode"

For context here is what the figma design looks like:

Linear task: https://linear.app/comm/issue/ENG-5959/search-users-ui-changes

Depends on D10746

Test Plan:
Please see the screenshot below

Reviewers: atul, inka

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10747