Stop using get_by!/2 in CodeCorps.ModelHelpers

[joshsmith]

When using get_by!/2, plug uses a different conn than our original one. This breaks CORS by not allowing us to render out our CORS headers.

While this behavior was nice to get started with, it is not tenable moving forward.

We can either solve this by using Canary's load_resource to render json-api errors where needed, or by rendering our own errors in the controller.

Either way, we need to switch to get_by/2 and handle the if/else condition appropriately.

[joshsmith]

get_by!/2 here:

web/models/model_helpers.ex:    query |> CodeCorps.Repo.get_by!(slug: slug |> String.downcase)