Repo/Listing.go: Refactor CreateListing and UpdateListing

[cpacia]

Both CreateListing and UpdateListing are nearly identical in repo/listing.go. The only difference is the check for whether the listing exists and whether a slug is created or not. It's likely these two functions can be refactored to avoid duplication and for clarity.

Also, the name of CreateListing is misleading because it is not persisting anything to disk. Only returning a new Listing object. Thus is might be better named NewListing and just be used as a constructor.

[cpacia]

Adding new refactoring tasks..

• There are a number of places where we create a pb.Listing and pass it around through a number of functions. We should find these areas and convert the pb.Listing to a repo.Listing and then refactor all functions in the trace that accept a pb.Listing to instead use the repo.Listing.

• These functions should make use of the repo.Listing methods where appropriate.