This adds HostingService.supports_list_remote_repositories, which is

used for determining if get_remote_repositories() and

get_remote_repository() can be called. This defaults to false, but

GitHub enables it.
The API exposes this on HostingServiceResource, and

HostingServiceAccountResource uses it to determine whether to show a

'remote_repositories' link.

Unit tests pass.
Tested manually by looking at the API results for GitHub services/accounts

and non-GitHub services/accounts.