[web] Add callback adding members to thread in AddMembersModal
Summary: Introduce callback for dispatching server action in AddMembersModal. It is executed after selecting all users to add and pressing "Add selected members" button.
Test Plan: After pressing the button, all selected users should be added to the thread
Reviewers: palys-swm, atul, benschac
Reviewed By: palys-swm
Subscribers: ashoat, palys-swm, Adrian, atul, karol-bisztyga, benschac, yayabosh
Differential Revision: https://phabricator.ashoat.com/D3744