Define API layer

[c-w]

This pull request defines a boundary between business/domain logic (how do we go about storing and analyzing images) and the API layer (Flask specifics such as which REST and Auth frameworks are used).

This increases testability and makes it easier to understand the code since each layer has well defined responsibilities:

• Models are for data storage.
• The auth layer verifies access to the application independent of the web framework that is used.
• The domain layer implements the app's functionality.
• The API layer adapts between HTTP requests and business logic or authentication.