refs https://github.com/TryGhost/Team/issues/2416
This extends the mock API to use a more formal pattern of moving our
entity code into a separate package, and use the service/repository
patterns we've been work toward.
The repository is currently in memory, this allows us to start using
the API without having to make commitments to the database structure.
We've also injected a single fake webmention for testing. I'd expect
the Mention object to change a lot from this initial definition as we
gain more information about the type of data we expect to see.
