[web] Center modals vertically

Summary: According to the designs, modals should be centered vertically. The change removes static top margin and centers the modal component

Test Plan: In web app, all existing modals should display in the center of view.

Reviewers: palys-swm, atul, benschac

Reviewed By: atul, benschac

Subscribers: ashoat, palys-swm, Adrian, atul, karol-bisztyga, benschac, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3666