[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