This adds a new site config option to configure pinned repositories. An admin can manually specify which git repositories will be pinned to gitserver instances.
Please note that for now, this feature is experimental. The pinning works, but we'll need to follow up for janitor code that will smoothly migrate the cloned repo to another gitserver instance (so we avoid an additional re-clone). This is currently in progress. After a conversation with @asdine, we agreed that it's easier to reuse this code once ready (instead of developing the same thing twice).
