[web] move the send friend request button to add users list modal

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

With the new modals API introduced in D10228 we need to move + lift the logic for the send friend request button to the AddUsersListModal so that we can pass it in as the primaryButton prop

For context here is what the action button container for the add users modal should look like (subsequent diffs will handle more updates to the UI):

Depends on D10344

Test Plan:
flow + checked that the confirm button for the add friends modal visually follows the new modal rules:

Reviewers: atul, inka

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10719