Refactor IActivity
Add IDomainEvent and remove ActivityVisibleAttribute, now activites can only bind to domain events
Docs for events
Docs for activity
Design notes for Activity
Refactor promotion
Docs for promotion
Test syntax highlight markup in markdown docs
Add syntax highlight to docs and improve event docs
Syntax highlight for activity docs
Refactor payment processors
Docs for payment processor
Refactor shipping rate provider
Docs for shipping rate provider
Remove IShipmentTracker

- According to the discussions before, we don't implement processes after payment in the core, so shipment trakcing will not be used in the core
Remove the old Kooboo.Commerce.WebAPI project because it's been moved to Kooboo.Commerce.Web
Add BatchedQuery to replace QueryStream
Minor refactoring
Minor
Add special events to Events doc