Story 920: Show uploads tutorial after following anon cta

Enable the anonymous user CTA in beta.
Refactor photo uploads tutorial css to be more generic
Create ContentOverlay - an overlay designed to appear within the page
rather than over the page

Currently shown to all users after logging in to make testing
easier

Note: currently doesn't work with the CentralAuth intermediate
screen of death (as this steals article_action=photo-upload from
query string)

Change-Id: Ic54af4d87553218b2e382544ee852508717bdf39
Merge "Story 920: Show uploads tutorial after following anon cta"