text: Title: Modify app template to use `table_finder`
  Body: When using `piccolo app new my_app` to create a new Piccolo app, the `piccolo_app.py` file generated doesn't use `table_finder` (note below how `table_classes=[]`).
  
  https://github.com/piccolo-orm/piccolo/blob/eb6ba32e85c2357969e0131a2991b7e50a1dcfb1/piccolo/apps/app/commands/templates/piccolo_app.py.jinja#L14-L20
  
  If you look at what we do with the `piccolo asgi new command`:
  
  https://github.com/piccolo-orm/piccolo/blob/eb6ba32e85c2357969e0131a2991b7e50a1dcfb1/piccolo/apps/asgi/commands/templates/app/home/piccolo_app.py.jinja#L14-L22
  
  Note how `table_classes=table_finder(modules=["home.tables"], exclude_imported=True)`.
  
  Using `table_finder` is easier, as it means you don't have to manually register the tables for migrations to work. Sometimes people might want to do it manually, but doing it automatic is preferred in most situations.