Implement database CRUD for all entities #10
Description
This will be implemented using a trait or set of traits in tavern_server to prevent coupling Tavern's requirements to the generic-use library. The functions will take a (mutable?) reference to a database connection or connection pool and perform the intended operation. Any errors should be returned via the Result type, using the custom error type.
All operations on all types should also be unit tested, as must as possible using a database connection. These tests may need to be implemented as integration tests instead.