Simple CQRS implementation based on:

• https://gist.github.com/pcreux/d094affd957a336af4f59b85f6ec0e6d
• https://kickstarter.engineering/event-sourcing-made-simple-4a2625113224

• cleanup aggregate definitions
• add more tests
• improve changeset validations
• add helpers for changeset based validations for commands
• remove hard coded reference to the repo'
• aggregate locking

# 1st terminal: plis run postgres
# 2nd terminal: mix test.watch