Add tests

[allisonking]

Also does some refactoring

• Moves Wackabase out of Etsy. Instead, it's instantiated by wack.py and passed around from there.
• Removes references to etsy oauth
• Attempts to add a gh workflow
• Separates out num_sales into its own file. I think we can think of files as separate tables in a db, so it makes sense for this to be its own lil file