[web] lift primary button logic from AddMembersModalContent into AddMembersModal

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

To follow the conventions of the new Modal api introduced earlier in this project, we need to lift the logic for the primary button to the same level where we render SearchModal so we can pass the button in as the primaryButton prop

Linear task: https://linear.app/comm/issue/ENG-5976/add-members-modal-experience

Depends on D10943

Test Plan:
Please see demo video below (please note there are some weird UI stuff that still needs to be polished up but a subsequent diff will fix that)

Reviewers: atul, inka

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10944