[web] Open delete account modal after Delete Account button is clicked.

Summary:
Show a modal when user clicks the button.

Depends on D4119

Test Plan: Click the button and check if the modal was displayed. Close it by clicking outside and by clicking x button.

Reviewers: def-au1t, atul, benschac

Reviewed By: atul

Subscribers: ashoat, Adrian, yayabosh

Differential Revision: https://phabricator.ashoat.com/D4120