Move fee rule handles and labels to one location

[kennethormandy]

• [ ] Decide on fee rule handles, currently:
  • price-percentage
  • flat-fee
• [ ] store them as a constant or something in the plugin. Should also be establishing the labels in the same place and passing that into the templates, but that can also be a separate issue for later

Also if the label has a currency symbol in it, will need to default to the Commerce default currency probably.