Ability to import realistic sample data with one click

[aldeed]

This is an updated restatement of https://github.com/reactioncommerce/reaction/issues/1864

In 2.0.0, there is a sample dataset loaded on startup if the database is empty. I propose that we do not do this. Instead:

• Create a GraphQL mutation, loadSampleCatalog, which creates Products, Tags, Catalog, Navigation, Media, etc. for a demo catalog.
• Use the catalog sample data already used internally, but save it as JSON and image files in the API codebase.
• The mutation must do nothing if there are already catalog items / products in the database (avoid overwrite in production).
• The mutation must require shop admin or createProduct permissions.
• After this mutation is available, Catalyst can show a "Load Demo Catalog" button on the Products page when there are no products for a shop.