Change repo filtering for users to use a user ID reference, rather than the username

While this means we need an additional query for initial lookup, it makes the *filtering* query (which is the heavy part) require far fewer joins, thus making it more efficient.

Also adds a new unit test to verify that our filter filters to the correct set of repositories.

data/model/image.py

@@ -12,12 +12,18 @@ from data.model import (DataModelException, db_transaction, _basequery, storage,
                         InvalidImageException)
 from data.database import (Image, Repository, ImageStoragePlacement, Namespace, ImageStorage,
                            ImageStorageLocation, RepositoryPermission, DerivedStorageForImage,
-                           ImageStorageTransformation)
+                           ImageStorageTransformation, User)
 
 from util.canonicaljson import canonicalize
 
 logger = logging.getLogger(__name__)
 
+def _namespace_id_for_username(username):
+  try:
+    return User.get(username=username).id
+  except User.DoesNotExist:
+    return None
+
 
 def get_image_with_storage(docker_image_id, storage_uuid):
   """ Returns the image with the given docker image ID and storage uuid or None if none.
@@ -273,7 +279,8 @@ def find_create_or_link_image(docker_image_id, repo_obj, username, translations,
                           .where(ImageStorage.uploading == False,
                                  Image.docker_image_id == docker_image_id))
 
-  existing_image_query = _basequery.filter_to_repos_for_user(existing_image_query, username)
+  existing_image_query = _basequery.filter_to_repos_for_user(existing_image_query,
+                                                             _namespace_id_for_username(username))
 
   # If there is an existing image, we try to translate its ancestry and copy its storage.
   new_image = None