This standardizes on RemoteRepository as the name, which matches the

name of the upcoming API resource. It also adds some new fields to the

resource for storing the unique (per-service) ID of the repository and

the HostingService and account that the repository is linked to.
While in here, I noticed that the wrong field was being used to fetch

the repository's URL for GitHub. This updates to use `clone_url` instead

of `url`.

Unit tests pass.
Tested this by hand with the upcoming APIs.