Basic labels support

Adds basic labels support to the registry code (V2), and the API. Note that this does not yet add any UI related support.

data/model/__init__.py

@@ -5,9 +5,18 @@ class DataModelException(Exception):
   pass
 
 
+class InvalidLabelKeyException(DataModelException):
+  pass
+
+
+class InvalidMediaTypeException(DataModelException):
+  pass
+
+
 class BlobDoesNotExist(DataModelException):
   pass
 
+
 class TorrentInfoDoesNotExist(DataModelException):
   pass
 
@@ -105,7 +114,23 @@ config = Config()
 
 # There MUST NOT be any circular dependencies between these subsections. If there are fix it by
 # moving the minimal number of things to _basequery
-# TODO document the methods and modules for each one of the submodules below.
-from data.model import (blob, build, image, log, notification, oauth, organization, permission,
-                        repository, service_keys, storage, tag, team, token, user, release,
-                        modelutil)
+from data.model import (
+  blob,
+  build,
+  image,
+  label,
+  log,
+  modelutil,
+  notification,
+  oauth,
+  organization,
+  permission,
+  release,
+  repository,
+  service_keys,
+  storage,
+  tag,
+  team,
+  token,
+  user,
+)