Payment signals BOOK-741

- setup the status mappings for order, order_payments and payments
- added FSM for three components
- created signals for the three components
- added some basic tests
- updated migrations to remove references to abstract classes
- added start of guard file for watching tests
Apparently the order of mixin and base class doesn't matter. I thought it did and that mixins should come first, but googling around it appears you can do it either way. I couldn't find any preferred way so please ignore my previous comment on this.