Convert Dataset and Org to events

[jeffgrunewald]

As a Smart City developer
I want my new datasets and organizations to be treated as events
So that I can respond to their changes over time

ACs:

• Struct definitions are migrated from smart_city_registry to smart_city repo
• smart_city_registry refactored to make calls to the new structs instead of the old ones for backward compatibility of apps still using registry

Tech Notes:

• Share creation logic of structs from maps between the different event types (struct from json string, struct from string-keys map and atom-keys map)
• Event names should be DatasetUpdate/OrgUpdate or DatasetUpsert/OrgUpsert to express unified create/update functionality