Add ability to unlink a layer from a repository.

Add Blobs() to Registry.
Add BlobService with the ability to Delete() a blob.

Add support for custom routes and custom auth records per route.

Implement Delete in the manifestStore.

Add annotation to images indicating they're from a registry managed by
OpenShift. This makes determining if an image is prunable significantly
easier.

Remove registry URLs pruning configuration, as the above annotation
makes it no longer needed.

Have the registry send a response after attempting to delete layers.

Have the pruner process the registry's delete layers response.

Add more test coverage.

Update layer deletion responses to include per-layer errors.

Remove fronting mux router and use route and auth extensions added
upstream in registry.

Move admin registry handlers into pkg/dockerregistry/server.

Rename --older-than to --keep-younger-than and make it a time.Duration.

Fix tests from prior refactoring

Add TODO about pruning manifests (signatures) from the registry.

Update STI->Source for build strategy.

Because images are deleted from OpenShift via pruning, when we ask the
registry to delete a manifest (really just its signatures in storage),
our repository middleware simply needs to delegate the deletion to the
original distribution.Repository, as the image will have already been
removed from etcd by this point in time.