text: Title: `AppRegistry` should provide a warning if an app doesn't end in `piccolo_app`
  Body: In your `piccolo_conf.py` file you have an `AppRegistry` - something like this:
  
  ```python
  APP_REGISTRY = AppRegistry(
      apps=[
          "home.piccolo_app",
          "blog.piccolo_app",
          "profiles.piccolo_app",
          "piccolo_admin.piccolo_app",
      ]
  )
  ```
  
  I think it's tempting for someone new to Piccolo to just put the name of the app in, and not the path to the `piccolo_app` file.
  
  ```python
  APP_REGISTRY = AppRegistry(
      apps=[
          "home",  # WRONG! Should be "home.piccolo_app"
          ...
      ]
  )
  ```
  
  We could put in some kind of warning if the string doesn't end in `piccolo_app`, or if the import fails, automatically try again with `app_name.piccolo_app`.