[web] Create the modals that will allow the user to confirm role deletion
Summary:
This should be the final stack in the entire roles project. This is the client-side logic for web to handle role deletion. Per the designs, the warning message is the same as on native, with
either a generic deletion message if nobody is assigned the role or a more specific one if members have this custom role.
Depends on D8624
Test Plan:
Verified that the modal appears as expected for roles with and without members, and pressing the delete button actually deletes
Reviewers: atul, ginsu
Reviewed By: atul
Subscribers: tomek, ashoat
Differential Revision: https://phab.comm.dev/D8625