[web] Only render Loading component in SSR

Summary: This addresses ENG-2513. Now that the web app needs to wait on redux-persist before hiding Loading, all we need to render on the SSR side is Loading.

Test Plan: Load the site in both dev and prod modes and make sure there are no warnings about SSR not aligning with client-side rendering, and that there is no "double-render" effect

Reviewers: ginsu, michal, atul, tomek

Reviewed By: ginsu, michal, tomek

Differential Revision: https://phab.comm.dev/D5983