[web] Introduce Loading component to web

Summary:
Designs are from DES-8; see Figma animation here. This is the first step to fixing SSR on web... basically we need to align keyserver/web on some loading HTML.

This can be used in other places, but for now I'm introducing it to use while redux-persist is loading.

In a subsequent diff, I'll pass it down via SSR.

Test Plan:

Reviewers: ginsu, michal, atul, tomek

Reviewed By: ginsu, michal, tomek

Differential Revision: https://phab.comm.dev/D5981