Retun either {:ok, _} or :not_found when calling Repo.get(id)

[pedro-gutierrez]

Is your feature request related to a problem? Please describe.
At the moment, every time we ask for a model by id, we return turn the result of doing Repo.get(id) and this returns either a struct or nil. While this is fine, it is not so easy to pattern match in the caller code.

Describe the solution you'd like
Return a tuple {:ok, term()} when the model is found. Return :not_found when no such a row exists in the database.