Design - https://trello.com/c/S7q0A4P8/119-shareafact-onboarding-png
The first time a user goes to a page that has a lead image, display little popup saying something like "Hey, try our new visual sharing feature?" and shows a little Tweet A Fact card for that page (we may want to remove that afterwards)
Dependency: https://phabricator.wikimedia.org/T89987